Using ultrafast extreme-ultraviolet pulses fortime-resolved dynamics of molecules
chemisorbed on metal surfacesby
Chi-Fong Lei

A dissertation submitted in partial fulfillmentof the requirements for the degree ofDoctor of Philosophy(Electrical Engineering)in The University of Michigan
2003

Doctoral Committee:
Professor Henry C. Kapteyn, Chair
Professor John GlandProfessor Margaret M. Murnane
Professor Stephen Rand

cChi−Fong Lei
All Rights Reserved

2003For Ruth, Matthew and Jacobii

ACKNOWLEDGEMENTS

I would like to thank Profs. Henry Kapteyn and Margaret Murnane for their guidance
and patience during my six years of graduate study. They are excellent advisors and haveshown me how to tackle problems that I have never seen before, the defintion of scientificresearch. Their insight and knowledge at scientific problems have opened my eyes and led
me to the vast world of physics. I especially thank them for giving me opportunities totry many different things, and providing good advices when needed. I also appreciate thatthey have given me freedom to apply my ideas to the experiments, never forcing me to dosomething that I did not want to. I thank Henry for his sometimes “nit-picky” requirements– requiring me to not only get the design working, but to also design it cheaply. I think
this is good training for my future research – it is important not only to get the experimentdone, but to do it well.
I thank Dr. Michael Bauer for his guidance and for bringing me to the field of surface
physics. We had many good times doing these experiments, even while staying up verylate in the laboratory. Not only did he have brilliant ideas regarding the experiments, he
was also an excellent teacher. I would not have come this far if I had not working withhim all these years. Furthermore, thanks for his encouragements which kept me going
forwards when the experiments were not running well.
Thanks to Dr. Kendall Read for her valuable discussions about the experiments. Even
though we worked together for just a couple of months, I appreciate her patience in explaining the fine details to me, when I was very junior and have absolutely no idea what Iiii

was getting into.
I also want to thank many folks in the laboratory. Many thanks to Dr. Sterling Backus,
Ron Tobey, and Dr. Lino Misoguti for their help on the laser systems, without their help
these experiments would have been impossible. I would also like to thanks Prof. John
Gland, Prof. Steven Rand, Prof. Dan Dassau, Dr. Yi-De Chuang, Emily Gibson, Dr. Kyoungsik Kim, Dr. Robert Shelton, Dr. Randy Bartels, Prof. Zenghu Chang, Prof. Charles
Durfee, Dr. Eric Zeek, David Samuels, and Dr. Erez Gershgoren for their valuable discussions. Thanks to Dr. Yanwie Lui and Prof. David Attwood for making the high quality
multilayer mirrors for my setup. And also thanks to the people in the JILA machine shopand electronics shop for their excellent support and equipment making, including Seth
Wieman, Hans Green, Blaine Horner, David Alchenberger, Todd Asnicar, Paul Beckingham, Michael Whitmore, and James Fung-A-Fat. For the readability of this thesis, thanks
are due to David Samuels, Racheal Tearle, and Marilee DeGoede, who corrected my grammar and writing style.
Finally, I would like to pay the greatest tribute to my family. My wife, Ruth, has sacrificed very much to support me through this entire process, and very likely I cannot everhope to compensate her for her sacrificies. Thanks also to my mother, and my father- andmother-in-law for their support, both mentally and financially. Finally, my faith in Christianity has been vital to me, enabling me to survive some of the most difficult moments
during these years.

iv

TABLE OF CONTENTS

DEDICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ii

ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . .

iii

LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii
LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ix

CHAPTERI. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1
1.2
1.3

1

Desorption induced by intense ultrafast pulses . . . . . . . . . .
Surface chemistry induced by femtosecond pulses . . . . . . . .
Two-photon photoemission spectroscopy . . . . . . . . . . . . .
1.3.1 Image-potential states . . . . . . . . . . . . . . . . .
1.3.2 Electron inelastic decay dynamics on an adsorbate covered surface . . . . . . . . . . . . . . . . . . . . . . .
1.3.3 Atomic motion studied by 2PPE . . . . . . . . . . . .
Direct measurement of hot surface electrons induced by ultrafast
pulses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ultrafast optical methods for surface dynamics investigation . . .
1.5.1 Probing desorption dynamics with surface second harmonic generation . . . . . . . . . . . . . . . . . . . .
1.5.2 Time-resolved vibrational sum-frequency generation .
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5
8
10
11

II. Time-Resolved Extreme-Ultraviolet Photoemission Spectroscopy . . .

30

1.4
1.5

1.6

2.1
2.2

2.3

High Harmonic Generation . . . . . . . . . . . . . . . . . . . .
Time-resolved extreme ultraviolet photoemission spectrometer .
2.2.1 Pump - HHG Probe beam line . . . . . . . . . . . . .
2.2.2 Ultra-high Vacuum Chamber . . . . . . . . . . . . . .
2.2.3 Finding temporal and spatial overlap . . . . . . . . . .
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

v

15
19
21
24
24
26
28

31
39
39
44
50
52

III. Probing the transition dynamics of chemisorbed states of O2 on Pt(111) 53
O2 states on Pt(111) surfaces . . . . . . . . . . . . . . . . . . .
3.1.1 Physisorbed State . . . . . . . . . . . . . . . . . . . .
3.1.2 Superoxo and peroxo chemisorbed states . . . . . . .
3.1.3 Atomic Oxygen . . . . . . . . . . . . . . . . . . . . .
Experimental results of O2 /Pt (111) . . . . . . . . . . . . . . . .
3.2.1 Thermal Desorption Spectroscopy . . . . . . . . . . .
3.2.2 Static photoemission spectra of the different oxygenstates . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.3 Oxygen desorption induced by intense ultrafast pulses
3.2.4 Probing O2 /Pt(111) by TR-UPS . . . . . . . . . . . .
3.2.5 Permanent modification of O2 on Pt(111) . . . . . . .
Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1 Nonthermal surface electrons on Pt(111) surface . . .
3.3.1.1 Nonthermal surface electrons observed in
the experiment . . . . . . . . . . . . . .
3.3.1.2 Two temperature model and nonthermal
surface electrons lifetime . . . . . . . . .
3.3.1.3 Excitation of O2 by nonthermal surface
electrons . . . . . . . . . . . . . . . . . .
3.3.2 Mechanism of O2 excitation . . . . . . . . . . . . . .
3.3.2.1 Identification of the transient feature . . .
3.3.2.2 Excitation rate of molecular oxygen . . .
3.3.2.3 Energy transfer mechanism from nonthermal electrons to adsorbates . . . . . . . .
3.3.2.4 Possible molecular motion during state transition . . . . . . . . . . . . . . . . . . .
3.3.2.5 Decay channels for the excited state . . .
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

87
88
89

IV. Probing adsorbate oscillation on metal surfaces . . . . . . . . . . . .

91

3.1

3.2

3.3

3.4

4.1
4.2
4.3
4.4

4.5

Photoelectric current affects by molecular symmetry and EUV
polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Symmetry of the orbitals of CO/Pt(111) . . . . . . . . . . . . .
Resolving oscillations of adsorbed CO molecules . . . . . . . .
Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.1 CO normal modes on a platinum surface . . . . . . . .
4.4.2 Hot electrons mediated excitation . . . . . . . . . . .
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

53
53
54
58
58
58
60
63
65
70
71
72
72
73
78
80
81
84
85

92
96
100
104
104
110
113

V. Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

vi

5.1
5.2
5.3

Time-resolved core-level spectroscopy . . . . . . . . . . . . . . 115
Probing bimolecular chemical reaction on surfaces . . . . . . . . 116
Angle- and time-resolved photoemission spectroscopy . . . . . . 118

APPENDICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
A.1 Desorption induced by electronic transition (DIET) . . . . . . . 123
A.1.1 Resonant excitation by non-thermal surface electrons . 125
A.1.2 Desorption probability . . . . . . . . . . . . . . . . . 130
A.2 Desorption induced by multiple electronic transition (DIMET) . 133
BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

vii

LIST OF TABLES

Table
3.1

Comparisons of the adsorption energies and bond lengths between gasphase oxygen molecules (O2 ), superoxo (O−2 ), peroxo molecular oxygen2−(O2 ), and atomic oxygen (O) on fcc and hcp sites. . . . . . . . . . . .

56

Parameters of platinum(111) used in the calculation of electron and lattice temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

75

Character table of the symmetry group C∞v . (z is the direction of theprinciple symmetric axis) . . . . . . . . . . . . . . . . . . . . . . . . .

95

4.2

Table of the direct product (⊗) of the symmetry group C∞v . . . . . . .

96

4.3

Normal mode energies of CO adsorbed on Pt(111) (* calculated for CO/Ni)108

3.2

4.1

viii

LIST OF FIGURES

Figure1.1

1.2

Desorption yield of two-pulse auto-correlation for NO on Pd(111) surface as a function of the time delay between the two ultrafast pulses.
The laser fluences of the two pulses are varied as (a) 1.7 and 1.8 mJ/cm 2 ,(b) 1.6 and 2.2 mJ/cm2 , and (c) 1.3 and 3.0 mJ/cm2 . (d)-(f) Calculated
temperatures of the surface electrons and the lattice phonons. (figure
reproduced from Ref. [10]) . . . . . . . . . . . . . . . . . . . . . . . .

7Illustration of the CO/O/Ru(0001) reaction. (a) CO has a much lower
desorption barrier comparing to oxygen, which prohibit CO oxidation
with the O atom when excited thermally. (b) Under femtosecond pulse
excitation, the 1.8 eV barrier for O activation is overcome by couplingfrom the hot electrons, and CO2 can be formed. (figure reproduced from
Ref. [16]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10

1.3

(a) Schematic energy diagram illustrating the process of two-photon photoemission. (b) Image-potential in front of a metal surface. The figure
shows the wavefunction probabilities and the energy levels of the n = 1
and 2 image-potential states. The figure also shows the energy band gapof the metal surface, prohibiting the image-potential state electron from
penetrating back into the metal surface. (figure reproduced from Ref. [17]) 12

1.4

Two-Photo photoemission spectra showing the the image-potential stateof an Ag(100) surface at various time delays. The signal is maximized at
0 fs time delay, and gradually decreases with positive time delay. Inset(×100 vertical scale): the background spectra at +200 fs time delay due
to 2PPE by the single beam excitation (both pump and probe). (figure
reproduced from Ref. [19]) . . . . . . . . . . . . . . . . . . . . . . . .

ix

14

1.5

1.6

1.7

1.8

(a) Comparison of the full-width-half-maximum inelastic decay times
between a Cs covered (0.8 ML) Cu(111) surface and a clean Cu(111)
surface shows a remarkable time differences around the cesium unoccupied state between the Fermi edge and the vacuum level of Cu. Data
are taken by time-resolved 2PPE. (b) 2PPE spectra of a Cs covered anda clean Cu(111) surfaces clearly shows a Cs induced peak appears at
5.8 eV. The pronounced peak at 6.2 eV of the clean surface (dashedspectrum) arises from the occupied Shockley surface state. (figure reproduced from Ref. [22]) . . . . . . . . . . . . . . . . . . . . . . . . .

16

Projected surface band structure for (a) Cu(100) and (b) Cu(111). The
dashed line indicates an energy level of an excited alkali state at 2.6 eV
above Ef . The momentum distance to real bulk states from the band gap
center is much smaller for Cu(100) than for Cu(111). (figure reproduced
from Ref. [22]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

17(a) The 2PPE difference spectra give information on the dynamic surface
electronic structure. The 2PPE signal corresponds to one-photon absorption from both the pump and the probe beams, with the delay ∆ set to
an integral number of optical cycles. Dashed lines and solid vertical line
indicate the Gaussian peak deconvolution of SS and A, and energy of
A at ∆ = 0 fs, respectively. (b) EA (∆) from two independent experiments for θCs ∼ 0.09 ML. (c) The wave packet position RCu−Cs (∆)
and its polynomial fit. (d) Dashed and solid lines, respectively, represent the predicted and derived excited state PES near the ground-state
equilibrium geometry, and the dotted line represents the calculated antibonding state energy dependence on RCu−Cs . (figure reproduced from
Ref. [27]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

20Electron energy distribution function versus energy (reference to the
Fermi-edge) taken with the gold film excited by (a) 120 µJ/cm2 and(b) 300 µJ/cm2 absorbed laser fluence at various time delays. The
dashed line is the best Fermi-Dirac distribution fit with the corresponding electron temperatures are shown above the figures. All the figures are
drawn in log scale to better shown the hot electrons distribution above
the Fermi-edge. (figure reproduced from Ref. [30]) . . . . . . . . . . .

22

x

1.9

1.10

2.1

2.2

(a) Second harmonic signal versus CO exposure where the Cu(111) surface is under an ambient pressure of PCO = 5 × 10−8 torr. A single laser
shot with fluence of 4.5 mJ/cm2 to desorb CO molecules from the copper surface. Notice that the second harmonic signal decrease when thesurface has higher CO coverage. (b) Second harmonic probe signal versus time delay after CO desorption induced by an ultrafast pump pulse
with fluence F = 4.5 mJ/cm2 . The autocorrelation of the pump pulse isalso shown in the figure. Inset: Same data (solid line) on expanded timescale, along with two other fluences. (figure reproduced from Ref. [34])25(a) SFG spectrum of the C-O stretching vibration of CO/Ru(111) at
340 K and the spectrum of the 150 fs broadband IR pulse (dashed line).
Left inset: illustration of time-resolved vibrational sum-frequency generation. (see text) Right side: free induction decay of this vibration exhibiting exponential decay with τ = 580 fs, indicating a homogeneous
linewidth of 9.2 cm−1 . (figure taken from Ref. [35]) (b) Transient SFG
spectra of the C-O stretch vibration after excitation with 800 nm/100 fs
pump pulses with a fluence of 55 J/m2 , starting at a temperature of 340 K
and coverage of 0.33 ML. (figure reproduced from Ref. [36]) . . . . . .

27High harmonics generated in neon gas at 40 torr with a 1 ps duration,
1.053 µm Nd laser pulse at an intensity of 1.5 × 1015 W/cm2 . (figure
reproduced from Ref. [50]) . . . . . . . . . . . . . . . . . . . . . . . .

32

In the semi-classical picture, (a) the atomic coulomb potential is disturbed by the laser electric field, and the electron wave packet tunnels
through the weaken potential barrier to escape. (b) The electron wave
packet enters the free space “continuum” and continue to accelerate in
the laser field. On the subsequent half cycle, the electron is decelerated
and turns around to be accelerated towards the parent ion. The electronwith high kinetic energy then scatters with the parent ion to generatehigh harmonics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

34

xi

2.3

2.4

2.5

Schematic diagram of the hollow-core fiber used to generate the 27 th
harmonic (42 eV) probe pulse in this experiment. The driving ultrafast
beam is drawn in light color and the generated high harmonic beam isin dark color. The fiber has four sections, and the long one is the main
harmonic generation region. The noble gas is introduced to the capillarythrough the gap between the first and second, second and third sections(from the left). The design of having the first and the third short sections sandwiched between the second long section is to maintain a constant pressure at the second section where the high harmonic generation
happens. The fourth short section is optionally installed for differential
pumping purpose. The inner diameter for all the fibers is 150 µm. The
capillary length of all the short sections is 1 cm, and the length for thelong section is 8 cm. . . . . . . . . . . . . . . . . . . . . . . . . . . .

37

Schematic drawing of the time-resolved ultraviolet photoemission spectroscope. The pump beam is the beam line drawn at the lower part of
the figure; the probe beam is drawn at the upper part of the figure, and it
goes through the hollow-core capillary to generate high harmonics. The
main chamber is under ultra-high vacuum condition at all times (∼ 10 −10torr), and a gate valve is used to separate the main chamber from the restof the system. The multi-layer mirror chamber and the beam line areunder high vacuum (∼ 10−6 torr) condition. . . . . . . . . . . . . . . .

40

Combined reflectivity of the Si/Mo multi-layer mirror pair (2 mirrors)
used in the TR-UPS setup. The reflectivity of the mirrors are measured
by the synchrotron radiation facility in Lawrence Berkeley laboratory. .

43

2.6

The image of the 27th harmonic fluoresce at a phosphor screen. The
phosphor screen is mounted beneath the platinum sample, and the phosphor screen was raised to the sample position when this picture wastaken. The image was capture by a low-lux, high sensitive camera mounted
outside the UHV chamber and looking in through a view port. The spot
size of the harmonic is measured to be 400 µm. The spot seems elliptical because the phosphor screen is at 45 degrees with respect to theincoming EUV beam. . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

2.7

The output signal of the impedance matched MCP TOF detector when asingle electron hits the detector. The signal is directly measured from thedetector with an oscilloscope. It is very important that there is no signal
reflection or “ringing” after the main peak for high count rate electron
detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xii

47

2.8

2.9

3.1

3.2

3.3

3.4

3.5

3.6

3.7

Picture shows the internal arrangement of the TR-UPS chamber. In thisphoto, the sample is turned 90 degrees from the normal position for better viewing. Both of the pump-probe beams are coming in from theflange at the left side of the picture to reach the metal sample. . . . . .

49

Signal of second harmonic generation between the infrared pump andprobe pulses. The FWHM of the signal is 62 fs. . . . . . . . . . . . . .

51

Schematic diagram of the surface configuration of oxygen on Pt (111),2−
for the superoxo (O−2 ), peroxo (O2 ), and atomic (O) oxygen states. . .

54

TDS for Pt(111) covered with O2 : the rate of temperature increase is 3
K/sec from 77 K to 300 K; then 10 K/sec from 300 K to 1100 K . . . .

59

2−
Static photoemission spectra taken for the superoxo (O−2 ), peroxo (O2 )and atomic (O) oxygen states on Pt(111). . . . . . . . . . . . . . . . . .

62

Femtosecond laser-induced desorption for the two adjacent desorption
cycles with oxygen redosing between the two laser excitations. Insert
shows the integrated desorption signal for the two cycles. . . . . . . . .

66

Electron energy distribution at Fermi edge of O2 /Pt(111), for time delaysof 0 fs, 150 fs, 250 fs, and 2 ps. Nonequilibrium hot electrons can beseen at 0 and 150 fs; however, at 250 fs, the energy distribution has
thermalized. Inset: Fermi edges at 0 fs time delay (black square) andstatic spectrum without pump pulses excitation (hollow square), plottedin log scale. The electron distribution at 0 fs time delay can be fit to a
100 K Fermi-Dirac distribution with 6% of the electrons excited by thepump photon energy (1.5 eV). . . . . . . . . . . . . . . . . . . . . . .

67

Time-resolved high-resolution photoemission spectra from a saturation
layer of molecular oxygen adsorbed on a Pt(111) surface at liquid nitrogen temperature: (a) no pump beam; (b) with pump beam, zero timedelay between pump and probe; (c) 250 fs time delay between pump andprobe; (d) 500 fs time delay; (e) repeat of (b), taken immediately after(d). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

68

Integrated amplitude of the characteristic 6 eV peak feature as a functionof pump-probe delay. A fit to the data indicates an onset time of 550 ±140 fs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

69

xiii

3.8

3.9

3.10

3.11

3.12

4.1

4.2

Spectra taken during pump beam irradiation of the sample over a periodof 2 hours. During the first ≈ 50 min, the spectra are unchanged. However, after ≈ 100 min, the characteristic 6 eV peak gradually increases
and is permanently present on the subsequent spectrum taken. . . . . . .

71

Calculated electron temperature (Te ) and lattice temperature (Tp ) using
the “two-temperature model” at the platinum (111) surface excited by
a 10 µJ/mm2 , 55 fs ultrafast infrared pulse. The measured nonthermal
electron integrated intensity (square) is also shown in the figure for comparison purposes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

76

Integrated intensity of the peroxo signature peak compared to the rescaled(a) electron and (b) lattice temperature. . . . . . . . . . . . . . . . . . .

79

Because of the increased population of the surface electrons above theFermi level induced by the ultrafast pump pulse excitation, the surface
electrons will start to tunnel to the 1πg∗ orbital of the oxygen moleculeand this leads to energy transfer to the oxygen center-of-mass. . . . . .

86

Possible reaction diagram of how oxygen molecules react after excitedby an intense ultrafast pulse on the platinum surface . . . . . . . . . . .

89

Schematic diagram showing the bonding character of a CO molecule adsorbed on a metal surface. (a) The CO-metal bond is provided by themost extended orbital of the molecule which is the 5σ orbital. Charge
back donation from the metal into the formerly empty 2π ∗ orbitals is
indicated by thin arrows. (b) Schematic energy level diagram of the COmetal system. The energy levels on both sides are for the isolated CO
molecule and the metal surface. The middle diagram illustrates the energy levels of an adsorbed CO molecule. Band splitting for the adsorbed
CO happens for the 5σ and 2π ∗ levels due to hybridizaton with the metal
d-band and the p-orbitals. (figures reproduced from Ref. [92]) . . . . .

97

Photoemission spectrum of CO molecules adsorbed on a platinum(111)single crystal surface. Spectra are taken with high harmonic photons(42 eV) with different s (black) and p (red) polarizations. . . . . . . . .

99

xiv

4.3

(a) Schematic diagram illustrating that when the CO molecules are alignedwith the polarization of the EUV pulse, the photoemission cross-section
is maximized. After excitation by an ultrafast laser pulse, the CO molecules
start to vibrate. When the CO molecules slightly non-parallel to the EUV
polarization, the photoemission cross-section is reduced. (b) Pump-probe
spectra close to the 4σ peak. These spectra are taken when the polarization of the EUV probe pulses is perpendicular to the platinum surface(p-polarized) , and the polarization of the excitation pump pulses are parallel to the surface (s-polarized) with 15µJ/mm2 /pulse fluence. Spectraof 0 fs and 400 fs time delays are shown for comparison, showing thatthe the 4σ peak in both height and area at 0 fs time delay are bigger thanthose at 400 fs time delay. . . . . . . . . . . . . . . . . . . . . . . . . 101

4.4

Integrated area of the 4σ peak of the CO/Pt(111) plotted at various timedelays, showing that the CO molecules are oscillating at the platinum
surface after pump pulse excitation. . . . . . . . . . . . . . . . . . . . 103

4.5

Pump-probe spectra near the Fermi-edge, showing that hot electrons are
rapidly excited when the ultrafast pump pulses excites the surface. They
subsequently relax in about 200 fs. There is a static component resulting
from a small residual 29th harmonic. . . . . . . . . . . . . . . . . . . . 105

4.6

Schematic representation of the normal modes of on-top and bridgebonded CO on platinum surface. The relative length of the arrows represent the movements of the carbon and oxygen atoms. (figure adaptedfrom Ref. [100] and Ref. [101]) . . . . . . . . . . . . . . . . . . . . . . 106

4.7(a) When an ultrafast pulse excites the platinum surface, hot electrons are
created on the surface. The hot electrons then tunnel to the 2π ∗ orbitalof the CO molecule. (b) Due to the bonding character to the platinum
surface of the 2π ∗ orbital, the C-O vibrational mode ν1 and the CO-Pt
vibrational mode ν2 will be induced. The frustrated translation mode
ν4 could either be directly excited by the hot electron, or subsequently
excited due to energy coupling between modes. . . . . . . . . . . . . . 110

xv

5.1

(a) Schematic drawing of “cold-target recoil-ion momentum spectroscopy”(COLTRIMS). A two-stage supersonic gas jet is installed to provide awell-defined atomic beam, which collides with an energetic ion-beam.
Helmholtz coils and resistive coated parallel plates are used to generatehomogeneous magnetic and electric fields to confine the recoil-ions andthe ejected electrons. Position-sensitive micro-channel plates (MCP) areused to resolve the kinetic energy and emission angles of the recoil-ionsand electrons. (figure reproduced from Ref. [113]) (b) The anode of theposition-sensitive MCP is wrapped by a fine copper-wire (delay-line) on
each dimension. When an electron strikes the anode, the position of theelectrons on the detector can be extracted by measuring the time difference of the electronic signals at both ends of each delay-line. The
position for each dimension can be determined independently. (figure
reproduced from Ref. [114]) . . . . . . . . . . . . . . . . . . . . . . . 120

A.1

Energy level diagram showing that the non-thermal electrons are energized rapidly by an intense ultrafast pulse. These “hot” electrons could
scatter into the LUMO state of the adsorbates to transfer energy to excite the molecular motions through Franck-Condon excitation scheme.
(figure reproduced from Ref. [79]) . . . . . . . . . . . . . . . . . . . . 127

A.2

Potential curves illustrating the process of Desorption Induced by Electronic Transition (DIET). (a) Intra-molecular electronic excitation involving bound-to-repulsive excitation; (b) Intra-molecular electronic excitation involving attractive intermediate state mediation. (figure reproduced from Ref. [79]) . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

A.3

Potential energy curves of center-of-mass translational motion of thechemisorbed ion (charge transfer from surface) with respect to the surface, showing wavepacket propagation throughout the time sequence involving the negative-ion LUMO resonance. The distribution of the final adsorbate states, between vibrationally excited bound (white) anddesorptive continuum states (black) is shown as P (ε) versus ε. (figure
reproduced from Ref. [79]) . . . . . . . . . . . . . . . . . . . . . . . . 132

A.4

Trajectories for motion on the ground and excited PES. The dotted lines
represent the total energy of the adsorbate complex, with vertical segments arising from Franck-Condon transitions and horizontal segmentsfrom conservative motion on a given PES. (a)(b) Trajectories displayed
associated with a single-excitation DIET process. (c)(d) Trajectories forthe multiple-excitation DIMET process. (figure reproduced from Ref. [11])134

xvi

CHAPTER I

Introduction

Surfaces is known to be a very difficult physical system to study, due to tedious surface
preparation procedures, and the requirement for maintaining the surface in an ultra-highvacuum chamber to maintain a clean and well-defined surface. Despite these difficulties, surfaces are very important to our daily life, since many chemical reactions actually
happen on surfaces. Particularly, many metal surfaces are known to be very good catalysts for a large number of chemical reactions. Thus, understanding chemical reactions
on metal surfaces – or in particular, how gas molecules adsorb on metal surfaces and thenreact – is currently a “hot topic” for many chemical physicists worldwide. In the last two
decades, experiments designed to elucidate how gas molecules adsorb on metal surfaces,
and how they react at the molecular level, have achieved great success. By utilizing surface
sensitive techniques such as scanning tunneling microscopy (STM), low energy electron
diffraction (LEED), high resolution electron energy loss spectroscopy (HREELS), ultraviolet photoemission spectroscopy (UPS), x-ray photoemission spectroscopy (XPS), andhigh-resolution electronic microscopy, many surface processes have become much better
understood. However, experiments to directly observe dynamics of adsorbates on sur-1

2
faces, to understand the fundamental time scales for chemical reactions, and to understanddetailed dynamics, are still not generally available. This is because typical chemical reactions involve very high speed translational, vibrational and rotational molecular motions,characteristic of the activation energy for the reactants to create or destroy a chemicalbond. The typical time scale for a molecule to translate, rotate, or vibrate on top of a metal
surface is typically femtoseconds to picoseconds. For this reason, to observe the reaction
dynamics of a chemical reaction in real time, surface techniques with sub-picosecond, ultrafast time resolution will be required to take a “snap-shot” of a chemical reaction as it
happens.
Fortunately, ultrafast laser technology has advanced significantly in the last decade.
Laser pulses with sub-30 fs pulse width and terawatt peak power can now be routinely andeffectively generated in many laboratories [1]. Using these lasers, many different types ofpump-probe experiments have been implemented to investigate chemical reactions, especially in the gas-phase and the liquid-phase. These experiments have already demonstrated
many great successes. Pump-probe transient absorption spectroscopy and time-resolved
raman spectroscopy are examples of successful and widely used techniques. However,these techniques cannot be directly applied to the investigation of adsorbate/surface chemical reactions. In general, since there is only a single mono-layer of active molecules
adsorbed on the metal surface, surface-sensitive techniques with high sensitivity must be
employed – otherwise, signals coming from the bulk material underneath surfaces will
totally overwhelm the minute signals originating from the surface molecules.
Ultraviolet photoemission spectroscopy (UPS) has been extensively used for surface

3
studies, due to its excellent surface sensitivity. Typically, photons (energy range of 20
to 200 eV) generated either by synchrotrons or by laboratory-based light sources – suchas a helium lamp – are used to photoeject electrons from a surface. Light in this energy
range is called “extreme ultraviolet” or EUV. The reason for the high surface sensitivityof this technique stems from the fact that photo-emitted electrons in this kinetic energy
range have an extremely short penetration depth for materials – typically about ∼ 10 Å, orseveral mono-layers. Therefore, the electrons coming from the bulk will re-scatter back tothe bulk and never escape from the surface, and only electrons originating from the surface
escape it to be detected [2].
However, UPS techniques that use conventional EUV sources generally cannot be used
to observe molecular dynamics. This is because even the-state-of-the-art synchrotron radiation sources emit light in “long” duration pulse of several tens of picoseconds, and helium
lamps have no pulse structure. Thus, these light sources cannot be used to observe chemical reaction dynamics on sub-picosecond time scales. Fortunately, recently developed
high-harmonic-generation (HHG) techniques can provide coherent EUV pulses with up to
several hundred eV photon energy, and with femtosecond or even sub-femtosecond pulse
duration [3, 4, 5]. High-harmonic generation is a high-order nonlinear process driven by
an intense ultrafast pulse focused onto a noble gas medium. The strong electric field of
the ultrafast pulse strips valence electrons from the atoms. The freed electrons are thenaccelerated by the strong electric field to very high speed. When the electric field reverses
its direction as the electric field of the lightwave oscillates, these electrons turn aroundand recollide with the atomic core. The kinetic energy gained during the electric field

4
acceleration is then emitted as EUV photons. Since the complete generation process happens within one optical cycle of the infrared ultrafast pulse, the generated EUV pulses are
typically shorter than the driving infrared pulse.
Therefore, by using these ultrashort EUV pulses, molecular dynamics on metal surfaces can be investigated by using so-called “pump-probe” techniques. Generally, a pumpprobe experimental setup uses at least two laser pulses – one “pump” beam and one“probe” beam incident on the same spot on the surface. The pump beam initiates thephysical phenomenon under study. Then, the probe beam is sent onto the surface with atime delay with respect to the pump beam, to “probe” the system status at that moment.
This process is similar to taking stroboscopic pictures of the phenomenon at various times
after it starts. The pump and the probe pulses should both be faster than the changes ofthe system; otherwise, these “snap-shots” will be blurred. In this work, an intense ultrafast
infrared pulse will be the pump beam to trigger a surface system to evolve, and a EUV
pulse will be used to probe the status of the system through, using the technique of EUV
photoemission spectroscopy.
The use of ultrafast laser pulses to initiate a chemical reaction also results in a new andunique type of femtosecond laser-driven chemistry or “femtochemistry”. Surface chemical reactions driven by intense ultrafast laser pulses are different from conventional heatmediated chemical reactions, in that the product yield from ultrafast laser-driven reactions
is often much higher for a given amount of deposited energy. Also, otherwise unaccessible reaction pathways become possible using ultrafast pulse excitation. These findings
suggest that the underlying mechanisms for these chemical reactions are different for ul-

5
trafast laser driven reactions. Non-equilibrium or “hot” electrons are created by sudden
energy deposition – these hot electrons are likely the driving mechanism for these reactions, replacing the role of phonons in conventional surface chemistry. Furthermore, these
hot electrons can nearly instantaneously reach effective temperatures of several thousand
degrees while the phonons in the lattice remain cold. This “two temperature” situation differs for conventional phonon-mediated chemical reaction conditions, where the electronsand phonons have equal temperatures.
In this chapter, I give a brief overview describing the current status of the field of
ultrafast surface chemistry.

1.1 Desorption induced by intense ultrafast pulses
Molecular desorption from a metal surface is one of the most basic phenomena in surface chemistry. When the temperature of a metal surface increases, atoms or molecules
adsorbed on a surface will desorb. When an adsorbate-covered metal surface is irradiation by a continuous-wave (cw) laser or a nanosecond pulse laser, the desorption rateof adsorbates is a linear relation to the laser fluence. The traditional wisdom treats thisphenomenon as a heat transfer process that transfers energy from the metal surface to theadsorbates through photon scattering with the adsorbates. The typical time scale for theseprocesses is in the range of nanoseconds. Due to this “long” time scale, the metal surface
and the adsorbates maintain an thermal equilibrium state throughout the desorption process. In particular, the electron temperature and the lattice temperature of the metal surface
are equal to each other at all times during the heating process. Desorption happens solely
due to phonon scattering from the lattice to the adsorbates, with no role for the surface

6
electrons.
However, desorption induced by femtosecond or picosecond laser pulses can no longer
be explained by the photon scattering mechanism, because the desorption character isdrastically different from the conventional case. In the case of an ultrafast laser pulse
incidents on a metal surface, the desorption rate increases nonlinearly to the input laser
fluenceY ∝ Fn(1.1)

where Y is desorption yield, F is the laser fluence, and n is an experimental fitting parameter for the power-law – for example, for O2 /Pt(111), n = 5.6 ± 0.7 [6]; for NO/Pd(111),n = 3.3 [7]. Furthermore, compared to the conventional thermal desorption, the desorption cross-section – the number of molecules desorbed per laser photon incident –induced by femtosecond pulses is exceptionally high. For example, NO and O 2 both exhibit roughly comparable desorption yields on Pd(111) in the case of conventional heating(nanosecond pulses irradiation), with a cross-section σthermal of ∼ 10−22 cm2 . But when
excited by femtosecond pulses, the NO and O2 desorption yield increases tremendously,
up to 1% desorption of the entire adsorbate coverage can be desorbed with each laser pulse
incident. This corresponds to an effective desorption cross-section of σ f s ∼ 10−18 cm2 –a 4 order of magnitude increase [7].
In addition to the observation of a nonlinear increase in desorption yield with intensity, there are other unique characteristics of the femtosecond desorption process. The
translational and vibrational energy distribution of the desorbed molecules was found to
be characteristic of temperatures several times that of the lattice temperature. The desorp-

7

Figure 1.1: Desorption yield of two-pulse auto-correlation for NO on Pd(111) surface as afunction of the time delay between the two ultrafast pulses. The laser fluencesof the two pulses are varied as (a) 1.7 and 1.8 mJ/cm2 , (b) 1.6 and 2.2 mJ/cm2 ,and (c) 1.3 and 3.0 mJ/cm2 . (d)-(f) Calculated temperatures of the surface
electrons and the lattice phonons. (figure reproduced from Ref. [10])tion angular distributions of the ejected molecules is also strongly forward peaked (cos n θ,n2), compared with a conventional cos 2 θ distribution [7, 8, 9].
In an attempt to understand the new mechanisms behind femtosecond laser desorption,correlation experiments have been used [10]. In this scheme, two ultrafast pulses are
incident colinearly on the metal surface. The time-delay between these two pulses canbe varied. Since the desorption yield for femtosecond laser desorption is nonlinear, thelaser fluence of each pulse can be set low enough to avoid strong desorption from the8sample by a single pulse, but when the two pulses overlap in time, the desorption yield
increases. Measuring the desorption yield as a function of the time-delay between thetwo pulses yields interesting results, as shown in Fig. 1.1. The full-width-half-maximum(FWHM) of this desorption peak is on the order of 1 ps which is longer than the durationof the pulse, but still short. Thus, the characteristic time scale of the process involved in
the nonlinear desorption is ∼ps. This fast desorption time scale cannot be explained by
the conventional phonon-induced desorption, or even multi-phonon induced desorption.
However, calculations have shown that the temperature of surface electrons is heated much
faster than the lattice phonons, and the temperature of the surface electrons can reachseveral thousand degrees, while the temperature of the lattice phonons does not increasesignificantly. Additionally, the temperature of surface electrons also has a decay time
similar to that of the desorption peak, in contrast to the long (several tens of picoseconds)
decay time of the lattice phonons [10, 11, 12]. Therefore, “hot” surface electrons createdby the incident femtosecond laser is likely to be the underlying mechanism behind thisnovel desorption phenomenon.

1.2 Surface chemistry induced by femtosecond pulses
Chemical reactions on metal surfaces can also be induced by laser light. Similar tothe case of desorption, conventional chemical reactions on metal surfaces can usually be
explained by phonon-mediated mechanisms. For cw or nanosecond pulsed lasers, thereaction yield is generally linear proportional to the incident laser intensity. Furthermore,
the product yield depends on the incident laser wavelength; for example, for the CO +O2 + hν → CO2 + O on platinum (111) single crystal, the CO 2 yield is undetectable for9laser wavelengths longer than 550 nm with nano-second pulses [13, 14].
On the other hand, in the case of sub-picosecond laser pulse excitation, the results arevery different. For the CO/O/Pt(111) case, it is possible to induce reactions with 800 nm
light, and the production yield increases nonlinearly with pulse width and laser fluence;
i.e. when the laser fluence at 800 nm increases by a factor of two, the CO 2 production
increases by considerably more than a factor of ten [12, 15]. Additionally, the branchingratio of the desorption yield increases dramatically when sub-picosecond pulses are used:the desorption ratio of O2 to CO2 is about 10 for femtosecond pulses, in sharp contrast
to the ratio of 0.5 found using nanosecond irradiation [12, 14]. To understand this newobservation, two-pulse correlation techniques are also employed to investigate if thesenew phenomena can be accounted for by the involvement of hot surface electrons. For
CO2 , the cross correlation desorption peak has a FWHM of 1.8 ps, indicating the surface
electrons play an important role to this new type of reactions [12].
In additional to the nonlinear effects seen with femtosecond pulses, previously prohibited chemical reactions become possible in the case of ultrafast pulse excitation. Thisobservation is exciting in that it hints that by using ultrafast laser pulses, surface chemistry
can be manipulated. This control over surface reactions is a long standing goal of the field.
CO/O/Ru(0001) is a good example of this control [16]. It is known that CO cannot be
thermally oxidized on the ruthenium surface because the CO molecules desorb more easily from the surface than the O atoms, giving no opportunity for the oxidation to happen.
However, when the ruthenium surface is irradiated by ultrafast pulses, hot surface electrons induced by the ultrafast pulses now transfer to an non-occupied level of the oxygen

10

Figure 1.2: Illustration of the CO/O/Ru(0001) reaction. (a) CO has a much lower desorption barrier comparing to oxygen, which prohibit CO oxidation with the
O atom when excited thermally. (b) Under femtosecond pulse excitation, the
1.8 eV barrier for O activation is overcome by coupling from the hot electrons,
and CO2 can be formed. (figure reproduced from Ref. [16])atom, transferring kinetic energy to the center-of-mass of the oxygen atom and causing theoxygen atom to move and react with the CO molecules. This process is shown in Fig. 1.2.

1.3 Two-photon photoemission spectroscopy
Section 1.2 discussed that the interaction of femtosecond lasers with surfaces results
in a variety of new physical phenomena. However, femtosecond lasers can also help study
dynamic processes on surfaces. In particular, two-photon photoemission (2PPE) spectroscopy is the most versatile tool for studying electronic dynamics on metal surfaces.
The basic experimental setup for 2PPE is very similar to that for the two pulse desorption experiment described in Sec. 1.1, except that photoelectrons emitted from the sample
surface are detected, rather than desorbed molecules. Valuable electronic dynamics infor-

11
mation can be obtained by measuring the kinetic energies of these photoelectrons. These
data constitute photoelectron energy spectra corresponding to different time delays between the two incoming pulses. For the 2PPE experiment shown in Fig. 1.3(a), the photon
energy of the first ultrafast pulse is generally not high enough to liberate electrons into thevacuum; however, together with the second pulse, the combined photon energies are large
enough to overcome the work function of the surface. Therefore, when the first ultrafast
pulse arrives, electrons in the valence band are excited to an “intermediate” state – locatedbetween the Fermi edge and the vacuum level – by the first pulse, and the excited electrons
will stay at these intermediate levels with a lifetime τ . If the second ultrafast pulse arrives
within this lifetime τ , electrons residing in this intermediate level will be ejected from thesurface. Therefore, electron dynamics – and in particular the intermediate life time τ –can be measured by observing how the photoelectron energy spectra change with respect
to the time delay between the two ultrafast pulses.
1.3.1 Image-potential states
Many interesting physical systems can be studied using 2PPE. For example, imagepotential states have been extensively studied by 2PPE. An image-potential state is anenergy state located just below the vacuum level of the metal surface, where the imagepotential state electron is hovering above the metal surface, but is loosely bound. The
binding force can be understood by considering the image charge of opposite sign positioned behind the metal surface at a distance equal to the electron. The image charge isactually a mathematical equivalent of the charge redistribution accumulated on the metal
surface. Since the electron is loosely bound by the surface, the wavefunction of the image-

12

(a)

e-

(b)
n=1

metal
Ekin

n=2

hν

6 eV

Evac
Eim

band gap
Evac

3 eV

hν

Ef

vacuumimage
potential

Ei

Ef

Figure 1.3: (a) Schematic energy diagram illustrating the process of two-photon photoemission. (b) Image-potential in front of a metal surface. The figure shows the
wavefunction probabilities and the energy levels of the n = 1 and 2 imagepotential states. The figure also shows the energy band gap of the metal surface, prohibiting the image-potential state electron from penetrating back into
the metal surface. (figure reproduced from Ref. [17])13potential state electron can extend up to several ten Å from the metal surface, as shownin Fig. 1.3(b). Since these electronic wavefunctions exist nearly completely in the vacuum, these electrons spend most of their time in the free space, and the chance to scatterwith the surface for relaxation is greatly reduced. Therefore, the image potential states
are expected to be long lived relative to bulk states in metal at comparable excess energies. Image-potential states provides an opportunity to understand the dynamics of a
two-dimensional electron gas in a vacuum.
Since the nature of this coulomb potential binding the electron to the surface is that of
attraction to an “image” charge, the energy level structure is similar to that of the hydrogen
atom; i.e. an electron and a proton. The image-potential V (z) can be expressed as [17]V (z) =

−e2 1
4πε0 4z(1.2)

where z is the distance from the metal surface. This differs from the hydrogen atom
potential by only a factor of 4 in the denominator and the one-dimensional nature of thepotential. Therefore, the image states are similar to the Rydberg states, and the binding
energy (with respect to the Evac ) of the nth image state isEb (n) =Ry
0.85= 2 eV216nnn = 1, 2, · · ·(1.3)

A second reason for the long relaxation time of the image potential states is that the energy levels of these states usually lie within the projected surface band gap of a metal. Thisfurther pushes the electronic wavefunctions away from the surface. Therefore, the electron
has even fewer opportunities to scatter with the metal surface and relax. In general, fora high order image-potential state (n > 1), the wavefunctions extend much farther away

14

Figure 1.4: Two-Photo photoemission spectra showing the the image-potential state of anAg(100) surface at various time delays. The signal is maximized at 0 fs timedelay, and gradually decreases with positive time delay. Inset (×100 verticalscale): the background spectra at +200 fs time delay due to 2PPE by the single
beam excitation (both pump and probe). (figure reproduced from Ref. [19])from the metal surface, and the relaxation time for these states can be as long as several
hundred femtoseconds.
Inverse photoemission techniques can also be used to measure the relaxation lifetime of image-potential states, by populating the image states and measuring the intrinsic
linewidth Γ, which is related to the lifetime τ via the uncertainty principle Γ · τ = ~. Thismethod is particular useful in measuring the n = 1 image-potential state, for the typicalrelaxation time of the n = 1 state is rather short, i.e. Γ = 16 meV or the lifetime τ = 41 fs
for the n = 1 state on Cu(111) [18]. However, for the higher order image-potential state,
the lifetime is much longer, which results in a much narrower intrinsic linewidth. Hence
inverse photoemission suffers from the minimum energy resolution of the electronic detectors to resolve the lifetime for these higher order states.

15
The first measurement of the lifetime of the image-potential state by using 2PPE wasdone on Ag(100) for the n = 1 state. The lifetime was estimated to be in the range of
15 − 35 fs, measured with a 55 fs pulse width ultrafast pulses [19]. This is shown in
Fig. 1.4, where the measured lifetime accuracy was hindered by the long pulse width ofthe laser pulses for the n = 1 state. For the n = 1 state on Ag(111) an upper limit for
the lifetime of 20 fs is estimated [20]. The lifetime of the n = 2 state of the Ag(100) is
considerably longer and could therefore be measured more precisely as 180 ± 20 fs [21].
1.3.2 Electron inelastic decay dynamics on an adsorbate covered surface
The previous section concludes that when electrons are excited to the image-potential
states of a clean metal surface, the relaxation lifetime of these electrons can be very long(∼ 100 fs). This is due to the loosely-bound characteristic of these states; with the wavefunctions of the excited electrons located far away from the metal surface (∼ 20 Å). Whenadsorbate molecules are introduced to cover the clean metal surface, electrons can also
be excited to an unoccupied resonance of these adsorbate molecules. However, theseresonance states of the adsorbates are different from the image-potential states that they
are very “localized”. The electrons are now no-longer able to extend far away from thesurface; rather, they are confined within the adsorbate molecules (several Å). Therefore,
electrons in these resonance states have plenty opportunities to scatter with the decay channels inside the metal, resulting in a short inelastic decay time (several fs) for these excitedelectrons.
It would be useful to understand the inelastic decay dynamics of electrons in this situation because, as mentioned in Sec.. 1.1, desorption induced by femtosecond pulses is

16

(a)

(b)

Figure 1.5: (a) Comparison of the full-width-half-maximum inelastic decay times between
a Cs covered (0.8 ML) Cu(111) surface and a clean Cu(111) surface shows aremarkable time differences around the cesium unoccupied state between the
Fermi edge and the vacuum level of Cu. Data are taken by time-resolved 2PPE.
(b) 2PPE spectra of a Cs covered and a clean Cu(111) surfaces clearly shows
a Cs induced peak appears at 5.8 eV. The pronounced peak at 6.2 eV of theclean surface (dashed spectrum) arises from the occupied Shockley surface
state. (figure reproduced from Ref. [22])17Figure 1.6: Projected surface band structure for (a) Cu(100) and (b) Cu(111). The dashedline indicates an energy level of an excited alkali state at 2.6 eV above E f .
The momentum distance to real bulk states from the band gap center is muchsmaller for Cu(100) than for Cu(111). (figure reproduced from Ref. [22])generally explained by hot electrons occupying these adsorbate states. Fig. 1.5(b) compares 2PPE spectra of a clean copper surface (dashed line), and a Cs covered Cu(111)
surface (solid line) [22, 23, 24]. It clearly shows that a Cs induced peak appears in the Cs
covered spectra at 5.8 eV energy. This peak is attributed to an unoccupied cesium statethat lives between the Fermi edge and the vacuum level.
M. Bauer et al. observed an exceptionally long inelastic decay time for this Cs state
when adsorbed to Cu (111) surface [22]. As shown in Fig. 1.5(b), the decay time of theexcited electron for the Cs covered Cu(111) surface is 74 fs, but the decay time of excitedelectrons in the same energy range for a clean copper surface is only 63 fs. The timedifference between these two measurements corresponds to the inelastic decay time of thisunoccupied state induced by the Cs atoms, where the excited electrons can be occupied.
Hence the decay time of this unoccupied Cs state is estimated to be 11 ± 3 fs [23, 25].
However, when a Cs atom adsorbs on the (100) surface of copper, this inelastic decay
time is reduced to 6 ± 4 fs. This discrepancy is generally explained by the difference inthe band gap of the two Cu surfaces. Fig. 1.6 shows the projected surface band structures

18
for the Cu(100) and Cu(111) surfaces.
It has been known that for an image-potential state, in the first approximation, thelifetime is inversely proportional to the penetration of the wavefunction of the electron into
the bulk [26]. Therefore, the closer the image-potential states are located to a band gap
edge, the shorter the lifetime. On the other hand, for image-potential states, they exhibit
quite a strong, nearly free-electron-like dispersion. This is because electrons in the imagepotential states are rather loosely bound above the surface and the excited electrons are free
to propagate along the other two dimensions of the surface. For a fixed excitation energy,
the excited electrons are, however, harder to couple to the bulk states at high k k value dueto the restrictions of momentum conversation. This prohibits the excited electrons fromrelaxing into the bulk, resulting in a long decay time.
For the unoccupied states of the alkali atom at low coverage, no dispersion of thisresonance state can be observed due to the localized property of these states. Therefore,in contrast to image-potential states, the decay process of the localized alkali excitation
into the bulk states can in principle – within the requirement of energy conservation –also occur at quite high values of kk . Thus, at sufficiently high kk values, real substratestates become accessible for decay. Therefore, the decay time of adsorbate resonances istypically very short. Furthermore, the increased inelastic lifetime from Cu(100) to Cu(111)
can be explained by the position of the projected surface band gap. The band gap at
kk = 0 down-shifts by approximately 3 eV from Cu(100) to Cu(111), as shown in Fig. 1.6.
Because of such a large band gap for Cs/Cu(111), it reduces the decay channels for theelectrons in the Cs resonance; and therefore, the relaxation time increases to 11 fs.

19
1.3.3 Atomic motion studied by 2PPE
Adsorbate atomic motion on metal surfaces can also be observed using 2PPE pioneeredby H. Petek and his coworkers [27]. When Cs adsorbed on a Cu(111) surface, the band
gap of the copper surface is between -0.85 and 4.1 eV (relative to the Fermi energy E f ).
This inhibits the decay of the electron on the Cs atom, as discussed in the previous section.
Therefore, the relaxation time for the excited Cs electrons is exceptionally long (∼ 11 fs)[22, 28]. For the Cs/Cu(111) system, an intrinsic occupied surface state (SS) is located at-0.39 eV below the Fermi edge, and an unoccupied Cs anti-bonding state (A) is located at
3.1 eV (in the limit of zero Cs coverage). According to the theory [29], the exact energy
level of the anti-bonding state A is strongly affected by the distance between the cesium
atom and the copper surface RCu−Cs , and when RCu−Cs increases, the energy level of A
decreases. By using this property, 2PPE spectra are taken in which the pump creates theadsorbate motions and the probe tests its evolution. By subtracting the reference 240 fs
spectrum, the 2PPE background component excited by the two separated, non interacting
pulses is removed so that the correlation effects are much more prominent. As shown in
Fig. 1.7(a), the maximum value of the photoemission signal shifts to lower energies withlonger delays, indicating the cesium atom is moving away from the copper surface.
Figure 1.7(b) shows the energy shifts of A at various time delays, by which R Cu−Cs canbe deduced by inverting the time-dependent orbital energy E A (∆) via the calculated dependence of the 6s + 6pz orbital energy on n RCu−Cs . The results are shown in Fig. 1.7(c),indicating a motion of the Cs atom of about 0.3 Å in 180 fs. Furthermore, the excitedpotential energy surface of the Cs atom can be extrapolated from this data. A fit of the

20

(a)

(b)

(c)

(d)

Figure 1.7: (a) The 2PPE difference spectra give information on the dynamic surface electronic structure. The 2PPE signal corresponds to one-photon absorption fromboth the pump and the probe beams, with the delay ∆ set to an integral number of optical cycles. Dashed lines and solid vertical line indicate the Gaussian
peak deconvolution of SS and A, and energy of A at ∆ = 0 fs, respectively.
(b) EA (∆) from two independent experiments for θCs ∼ 0.09 ML. (c) The
wave packet position RCu−Cs (∆) and its polynomial fit. (d) Dashed and solidlines, respectively, represent the predicted and derived excited state PES near
the ground-state equilibrium geometry, and the dotted line represents the calculated anti-bonding state energy dependence on RCu−Cs . (figure reproduced
from Ref. [27])21
RCu−Cs (∆) to the fourth-order polynomial gives a continuous function R(∆), which isdifferentiated to obtain the acceleration of the Cs atom a = ∂ 2 R(∆)/∂∆2 . The force
acting on the Cs atom, where F = ma, is then integrated, U = −experimental PES (Fig. 1.7(d))

R

F dR, to derive theThis experiment demonstrated that the 2PPE can be used to measure the atomic motion
on a surface – however, this technique is not universal enough to apply to other systems.
The success of using 2PPE techniques in measuring atomic motion relies on the exceptionally long lifetime of the Cs anti-bonding state (AA) on Cu(111) resulting from the metallic
band gap. As mentioned above, the typical lifetime of an excited electron in an intermediate state is rather fast (< 10 fs), and in general is not long enough to observe most atomic
motions on surfaces.

1.4 Direct measurement of hot surface electrons induced by ultrafast
pulsesExperimental data and theoretical modeling both suggest that the laser photons aremostly adsorbed in the substrate within a skin depth (100 Å) of the surface, rather thanbeing absorbed in the adsorbate layer, thus eliminating multi-photon adsorption as a significant contribution to the nonlinear phenomena observed in Sec. 1.1 and 1.2 [31]. Dueto the low heat capacity of electrons, theories had proposed that surface electrons canbe highly excited by ultrafast pulses, while the lattice remains relatively cool. However,direct experimental confirmation of these highly excited surface electrons was not available until the work of J. Boker et al [30, 32, 33]. In his experiments, he heated a gold
film with 1.84 eV photons to generate hot surface electrons. A second laser pulse was

22

(a)

(b)

Figure 1.8: Electron energy distribution function versus energy (reference to the Fermiedge) taken with the gold film excited by (a) 120 µJ/cm2 and (b) 300 µJ/cm2
absorbed laser fluence at various time delays. The dashed line is the best
Fermi-Dirac distribution fit with the corresponding electron temperatures areshown above the figures. All the figures are drawn in log scale to better shown
the hot electrons distribution above the Fermi-edge. (figure reproduced from
Ref. [30])23frequency-doubled to 337 nm and subsequently frequency-mixed again with the 674 nm
to generate 5.52 eV photons. The work function of the gold film is only 5.1 eV, and hence
the 5.52 eV photons will have high enough photon energy to photoeject electrons close to
the Fermi-edge.
Figure 1.8 shows the energy spectra near the Fermi-edge of the gold film at various
pump-probe time delays. By first normalizing the spectrum to the joint density of statesof the gold film, the resulting spectra fits nicely with a Fermi-Dirac distribution function
at various temperatures. The results indicate that the surface electrons can be excited byultrafast pulses to very high temperatures (thousands of K), and this temperature cools
rapidly through relaxation with the lattice in a relatively short time (within 1 ps). In addition, for long time delays, the data can be fitted nicely by the Fermi-Dirac function. However, for time delays within the first one picosecond in particular, significant discrepancies
happen at the energy region above the Fermi-edge where “hot” or nonthermal electrons
exist. Because of the rapid excitation process, these hot electrons cannot be describedby a Fermi-Dirac energy distribution, which is only valid when the physical system is inthermal equilibrium.
The creation of these hot electrons can be explained by the the rapid heating of ultrafast
pulses, which rapidly increase the energy of the excited electrons by a value corresponding
to the photon energy. Since the electron relaxation time is longer than the excitation pulse,
the hot electrons will take some time before they can equilibrate to a Fermi-Dirac distribution. When these highly-excited hot electrons tunnel to the unoccupied states of theadsorbates, they subsequently provide energy for the nonconventional “femtochemistry”

24
observed on metal surfaces.

1.5 Ultrafast optical methods for surface dynamics investigation
Purely optical methods can also be applied to investigate chemical dynamics on metal
surfaces. These methods are particularly useful to understand the bonding character ofadsorbates, and the vibrational modes of the adsorbate-surface interaction. In this section,
two methods will be discussed that exemplify current developments in these investigations.
1.5.1 Probing desorption dynamics with surface second harmonic generation
In Sec. 1.1, two pulse cross-correlation techniques were used to identify if desorption
on metal surfaces is due to photo-induced or hot-electron-induced heating. This was doneby observing the cross-correlation width of the desorption peak. However, the timing
information of how long it takes for adsorbates to actually desorb from the metal surface is
not probed using this technique. J. A. Prybyla et al. have demonstrated that it is possible tomeasure this desorption time directly by using surface second harmonic generation. They
estimated the upper limit of the desorption time of CO adsorbed on Cu(111) to be 325 fs
when excited by an 2 eV photon energy, 100 fs pulse-width and 4.5 mJ/cm2 fluence pulse[34].
Figure 1.9(a) shows how the second-harmonic probe signal depends on a continuous
CO dosing rate (the ambient pressure is PCO = 5 × 10−8 torr). In general, the secondharmonic signal decreases as the CO coverage increases on the copper surface, with thesecond-harmonic signal becoming constant when the copper surface reaches the saturation
coverage. Then, a 100fs, 4.5 mJ/cm2 laser pulse desorbs the CO molecules from the

25

(a)

(b)

Figure 1.9: (a) Second harmonic signal versus CO exposure where the Cu(111) surface
is under an ambient pressure of PCO = 5 × 10−8 torr. A single laser shotwith fluence of 4.5 mJ/cm2 to desorb CO molecules from the copper surface.
Notice that the second harmonic signal decrease when the surface has higher
CO coverage. (b) Second harmonic probe signal versus time delay after CO
desorption induced by an ultrafast pump pulse with fluence F = 4.5 mJ/cm 2 .
The autocorrelation of the pump pulse is also shown in the figure. Inset: Samedata (solid line) on expanded time scale, along with two other fluences. (figure
reproduced from Ref. [34])26
copper surface. Because of the reduction of the CO coverage, the second-harmonic signal
increases, as expected. After CO molecules re-dose the surface to the saturation coverage,
the second-harmonic signal again reduces to the previous saturation level.
A pump-probe geometry has been setup to observe the desorption time-scale for the
CO/Cu(111) system, and Fig. 1.9(b) shows the corresponding results. The spike in thesecond-harmonic signal near zero time delay is due to the sum-frequency response of thepump-probe autocorrelation. The probe second-harmonic signal then falls to a constantbaseline at a time delay of 350 fs, which is persistent and at the same level of the Fig. 1.9(a).
In general, the second-harmonic response is due to both the desorption of CO moleculesfrom the surface and to electronic excitation of the surface. As soon as the electronic
excitation response is over, a constant baseline means the desorption event is also over,
implying that the upper limit for such the time scale of a desorption process is about
350 fs.
1.5.2 Time-resolved vibrational sum-frequency generation
Section 1.3 discussed that 2PPE is a very useful tool in determining the transient electronic properties and even adsorbate atomic motions. On the other hand, atomic/molecularvibrational information is also valuable in understanding chemical reactions. Vibrational
information, however, has not yet been obtained through this method or photoemission related techniques. Other methods are needed.1 The vibrational information can be obtainedby using surface sum-frequency generation (SFG) on CO/Ru(111), as demonstrated by G.
Ertl and his co-workers [16, 35, 36]. On metal surfaces, since the inversion symmetry1

In chapter IV, I will show that time-resolved ultraviolet photoemission spectroscopy can be used toobtain vibrational information.

27

(a)

(b)

Figure 1.10: (a) SFG spectrum of the C-O stretching vibration of CO/Ru(111) at 340 K
and the spectrum of the 150 fs broadband IR pulse (dashed line). Left inset: illustration of time-resolved vibrational sum-frequency generation. (seetext) Right side: free induction decay of this vibration exhibiting exponential
decay with τ = 580 fs, indicating a homogeneous linewidth of 9.2 cm −1 .
(figure taken from Ref. [35]) (b) Transient SFG spectra of the C-O stretch
vibration after excitation with 800 nm/100 fs pump pulses with a fluence of
55 J/m2 , starting at a temperature of 340 K and coverage of 0.33 ML. (figure
reproduced from Ref. [36])of the metal lattice is broken on the surface, the second-order non-linear susceptibility isnonvanishing at the metal interface of centro-symmetry materials [37, 38]. Hence, sunfrequency generation can occur at surfaces, and the measured nonlinear susceptibilites are
related to the molecular dynamics at the surface.
The left inset of Fig. 1.10(a) illustrates the basic idea behind time-resolved vibrational
sum-frequency generation (fs-SFG). An ultrafast pulse illuminates the adsorbate-surface
system to excite an adsorbate vibrational mode. The excited adsorbates then either desorb
from the surface, or vibrate under a free induction decay time to release its vibrationalenergy to the metal surface. Another pair of laser pulses subsequently probes the surface,
one of the two pulses is spectrally filtered to 4 cm −1 at 800 nm, and hence corresponds

28
to a long pulse (>ps). The other pulse is a broad band infrared pulse (spectrum shownin Fig. 1.10(a)) with a pulse width of 150 fs. The broad band infrared pulse serves as a“white light” source to scan for the frequency of the excited C-O stretching vibrationalmode, with the narrow band 800 nm pulse used for sum-frequency-generation of the SFG
signal. Since the resulting SFG signal (ωSF G = ωV IS + ωIR ) results from the multiplicityof the two pulses spectrally, it is also a narrow band signal, and hence it can be used to
identify any spectral shift of the C-O stretch mode due to ultrafast excitation. Temporally,
since the SFG generation only happens when the two pulses arrive on the surface, the pulse
width of the broad band infrared pulse determines the time-resolution of the probe SFG.
Therefore, adsorbate vibrational dynamics can be probed with femtosecond resolution.
The transient dynamics of CO on a Ru(111) surface, excited by 800 nm/110 fs pumppulses, is shown in Fig. 1.10(b), where the sample temperature is at 340 K, and the surface
is re-dosed using CO background pressure of PCO = 5 × 10−6 mbar. The C-O stretch
vibration displays a very strong transient red-shift, as well as a broadening and decrease inintegrated intensity. The red-shift is the result of anharmonic coupling of the C-O stretch
to low-frequency frustrated modes [39]. The intensity of the SFG signal at τ delay = 168 ps
is roughly half of the initial intensity, demonstrating that a large number of CO molecules
desorb after excitation by the pump pulse.

1.6 Conclusion
This chapter has presented a synopsis of work to date of observing adsorbate dynamics on metal surfaces. These techniques have revealed much valuable information to help
understand how adsorbates behave on metal surfaces on molecular time scales. The ulti-

29mate goal of tracing a complete chemical reaction process from the start to the end andidentifying all the intermediate states and their corresponding reaction time scales remains
to be done. Therefore, the development of new techniques to reveal complete adsorbate
dynamics is increasingly important in the field of surface chemical physics. In this thesis, I explore the potential of “Time-Resolved extreme Ultraviolet Photoemission Spectroscopy” (TR-UPS) to elucidate surface chemical reaction dynamics. Preliminary results
will be discussed that demonstrate the effectiveness of this technique for observing molecular motion on surfaces.

CHAPTER II

Time-Resolved Extreme-Ultraviolet Photoemission
Spectroscopy

In conventional surface analysis, x-ray photoemission spectroscopy (XPS) 1 and ultraviolet photoemission spectroscopy (UPS) are standard methods for analyzing metal surfaces. These methods have proven to be powerful in characterizing metal surfaces, and
UPS is especially important for obtaining information about molecules adsorbed on metal
surfaces. Photons in the extreme ultraviolet and soft x-ray regimes (50 to 200 eV) areof particular importance in probing metal surfaces. This is due to the fact that the mean
free path of photoemitted electrons excited by these photons is extremely short in metals(only about 5 to 10 Å); hence, only electrons originating from the top several mono-layers
will be detectable [2]. These photons can be generated using synchrotron radiation orlaboratory-size x-ray and ultraviolet light sources.
A long standing goal of chemical physics has been to follow a chemical reaction frombeginning to end, while identifying all the intermediate steps. However, conventional
UPS techniques are incapable of doing this due to the limitations of the light source used.
State-of-the-art – or so called third generation – synchrotrons cannot provide light pulses
1

In chemistry literature, XPS often is referred as Electron Spectroscopy for Chemical Analysis (ESCA).

30

31
shorter than tens of picoseconds duration, despite the high photon flux that can be obtained.
Since molecular motion (vibrational, rotational, and translational) on metal surfaces is
estimated to occur on sub-picosecond time scales, the development of innovative ultrafast
light sources that provides EUV pulses with femtosecond duration will provide crucial
tools for future investigations.
Several approaches for generating ultrafast EUV light are currently being pursued, including “slicing” synchrotron radiation bunches with femtosecond lasers, Thomson scattering from an electron beam [40, 41, 42], free-electron lasers (FELs) [43, 44, 45], andhigh harmonic generation techniques. FELs should be capable of generating EUV pulses
with sub-picosecond pulse width and extremely high intensity. Practical implementations
are still lacking, however, as these technologies are extremely difficult and expensive [46].
A very successful approach to obtaining ultrafast EUV pulses is high harmonic generation. Harmonic generation has a long history – optical frequency doubling was first
demonstrated by using a ruby laser and a quartz crystal in 1961 [47]. Since then, techniques for up-converting photons to higher energies have attracted significant attention.
In the last decade, the use of harmonic generation for EUV generation has been a topic
of extensive investigation, with the demonstration of very high (> 100 th ) order nonlinear
processes [48].

2.1 High Harmonic Generation
Ultrafast laser technology has progressed rapidly during the last two decades. In particular, the invention of the ultrafast Ti:Sapphire laser is of great importance [1, 49]. Not
only is it an all solid state laser system and therefore relatively easy to use, but furthermore,

32

Figure 2.1: High harmonics generated in neon gas at 40 torr with a 1 ps duration, 1.053 µm
Nd laser pulse at an intensity of 1.5 × 1015 W/cm2 . (figure reproduced from
Ref. [50])the laser pulses generated by Ti:Sapphire oscillators can straightforwardly be amplified using subsequent laser amplifiers. Therefore, table-top size, sub-terawatt class Ti:sapphirelaser systems have been constructed and are in use in many laboratories around the world.
When these intense laser pulses are focused into a gas medium, the interaction between
the strong optical field and the gas atoms produces strong nonlinear-optical effects. Oneof these effects is the production of optical photons with frequencies corresponding to the
odd harmonic orders of the original pulse. The wavelengths of these high harmonic generated (HHG) photons spread across the ultraviolet, the extreme ultraviolet, and the soft
x-ray regions of the spectrum. Furthermore, since the high harmonic generation process isa coherent process, the generated photons are highly coherent. This coherent property isimportant for many physical experiments.
A typical high harmonic generated spectrum produced by an intense ultrafast pulse
focused into a noble gas media is shown in Fig. 2.1. The major features of the spectrum
are a plateau at low photon energy, and a “cutoff” corresponds to the maximum achiev-

33able photon energy. Beyond the cutoff energy, the intensity of the HHG falls rapidly. The
emitted spectrum is not continuous, but features discrete harmonic peaks whose energies
correspond to the odd harmonic orders of the driving laser. The high harmonic generation process can be explained using a strong field laser-atom interaction model, in which
the linearly polarized electric field suppresses the electric field binding electrons to theinteracting gas atom [51, 52, 53]. This allows the atom’s valence electrons to tunnel outof its attractive potential and into free space, as shown in Fig. 2.2(a). The freed electrons
are then accelerated to a very high kinetic energy by the strong electric field of the ultrafast pulse. After a half optical cycle, the ultrafast pulse reverses its electric field, and theelectrons are forced to reverse their course. They may then recombine or scatter with theatomic core if their trajectory approaches the parent ion. The high impact energy of thiscollision causes them to release this kinetic energy into a high energy photon, as shownin Fig. 2.2(b). Since the electrons undergo this process in a very short timespan, the duration of the generated pulses is equally short, on the order of a half cycle of the driving
pulse, although the total emission may consist of a train of such pulses. Furthermore, since
the electrons from different atomic cores undergo coherent acceleration from the driving
pulse, the generated photons are coherent in nature.
Due to the total symmetry of the gas atoms, the generated photons are the odd harmonics of the driving pulse, and extend to the cutoff frequency ωmax . Theory predicts a ωmax
corresponding to the acquired kinetic energy plus the atomic binding energy~ωmax ' Ip + 3.2Up(2.1)

where Ip is the ionization potential of the gas atom, and Up is the cycle-averaged kinetic

34

(a)

E-field

electronatomic coulomb potential(b)high harmonic photons

atom

accelerated electron

Figure 2.2: In the semi-classical picture, (a) the atomic coulomb potential is disturbed
by the laser electric field, and the electron wave packet tunnels through theweaken potential barrier to escape. (b) The electron wave packet enters thefree space “continuum” and continue to accelerate in the laser field. On thesubsequent half cycle, the electron is decelerated and turns around to be accelerated towards the parent ion. The electron with high kinetic energy thenscatters with the parent ion to generate high harmonics.

35
energy of the electron in an electric field E0 and frequency ω0 , generally called the pondermotive potential. Up can be written out asUp =e2 E022πcre IL
=
2
4mω0
ω0

(2.2)

where IL is the laser field intensity, and re = e2 /4π0 mc2 is the classical electron radius.
To achieve as high a cutoff energy as possible, noble gases, with their higher ionization potentials, are typically used in the high-harmonic generation process. For example, helium’s
ionization potential is 24.5 eV, and for a laser intensity of 1015 W/cm2 with a 800 nm wavelength, ω0 = 2.4 × 1015 rad/s and re = 2.8 × 10−13 cm, so Up ' 60 eV. Therefore, using
Eq. 2.1, the theoretical maximum generated photon energy is about 220 eV for those conditions. Currently, the record for high harmonic generation is the 229 th harmonic (2.7 nm
or 450 eV) [3].
To generate “bright” high harmonic pulses, harmonic photons generated at different
points in the nonlinear medium have to interfere constructively; otherwise, the generated photons will convert back to the fundamental wavelength. Therefore, extending the
medium length does not automatically result in higher output flux. To illustrate this point,
for second-order harmonic (2ω) generation in a nonlinear medium, the generated photon
intensity can be written as:
sin2 (∆k L/2)
= sinc2
I2ω ∝(∆k L/2)2



∆k L
2



(2.3)

where L is the length of the non-linear media, and∆k = 2kω − k2ω =2ω(nω − n2ω )
c

(2.4)

36
where kω (nω ) and k2ω (n2ω ) are the wave vector (index of refraction) of the nonlinear
medium at wavelengths ω and 2ω, respectively.
Since Eq. 2.3 is a sinc function, it reaches its maximum when ∆k = 0. Therefore, inorder to achieve maximum output intensity for a given harmonic, the wave-vectors (2 ofkω and a k2ω ) of the fundamental and harmonic light must cancel out. Eq. 2.4 expresses
this “phase-matching” condition for harmonic generation.
For second order harmonic generation, the phase matching condition is easy to achieve,
either by temperature-controlling the non-linear medium, or by using a nonlinear crystal
and rotating the crystal to the so-called “phase-matching” angle. The idea of both techniques is to achieve equal values for both nω and n2ω , in order to satisfy the phase-matchingcondition. However, phase-matching harmonic generation for high-orders is not so trivial
since one cannot use the material birefringence. Furthermore, in the case of HHG, themedium is unavoidably adsorbs much of the generated signal. Therefore, high harmonic
generation generally suffers from low output photon flux.
In a typical high harmonic generation setup, a pulsed gas nozzle creates a well-defined
region of noble gas inside a vacuum chamber. The gas nozzle pulses synchronously with
the incoming focused laser pulses to generate the harmonic light. The reason to use a pulse
valve to generate harmonic light is to avoid absorption from the noble gas. However, withsuch a design it is difficult to achieve high fluxes of the harmonics since harmonic photons
are generated in a very limited region with varying pressure.
An innovative design has been implemented in our laboratory using a hollow-corecapillary to achieve phase-matching conditions for high harmonic generation. Fig. 2.3

37
Noble Gas

VacuumNoble Gas

VacuumHigh
Vacuum

Gas Pressure

Position

Figure 2.3: Schematic diagram of the hollow-core fiber used to generate the 27 th harmonic(42 eV) probe pulse in this experiment. The driving ultrafast beam is drawn inlight color and the generated high harmonic beam is in dark color. The fiberhas four sections, and the long one is the main harmonic generation region.
The noble gas is introduced to the capillary through the gap between the firstand second, second and third sections (from the left). The design of having thefirst and the third short sections sandwiched between the second long section
is to maintain a constant pressure at the second section where the high harmonic generation happens. The fourth short section is optionally installed for
differential pumping purpose. The inner diameter for all the fibers is 150 µm.
The capillary length of all the short sections is 1 cm, and the length for thelong section is 8 cm.
shows a schematic illustrating the high-harmonic capillary setup. The long center piece ofthe hollow core capillary is filled with noble gas. The wave-vector k of the laser beam canbe written in the following form when it is traveling inside the capillary [4]k(λ) ≈

2π 2πP (1 − η)δ(λ)++ (1 − η)n2 Iλλ
u2 λ−P ηNatm re λ − 11 2
4πa

(2.5)

where the terms, from left to right, correspond to the contributions from vacuum, neutral
gas dispersion, non-linear refractive index, plasma dispersion, and waveguide dispersion.
The symbols P , η, Natm , δ(λ), re , n2 , I, a and u11 represent the pressure in atmospheres,the ionization fraction, the number density at one atmosphere, the neutral gas dispersion,
the classical electron radius, the nonlinear refractive index, the laser pulse intensity, theradius of the capillary, and u11 = 2.405 is the first zero of the Bessel function J0 . The

38
important point here is that the wave-vector k depends on the pressure P , driving pulse
intensity I, and the capillary radius a; therefore, the magnitude of the wave-vector k can
be controlled by changing these parameters. The phase matching condition for the q th
harmonic generation becomes∆k = kq − qkf= qk0 [n(λq ) − n(λ0 )]≈ qu211 λ0
2πNa+ Ne re (qλ0 − λq ) −[δ(λ0 ) − δ(λq )]
2
4πa
λa

(2.6)

Therefore, phase-matching for high-harmonic generation can be achieved by “confining”
noble gas molecules in a capillary. The high harmonic flux generated in this geometry is
estimated to be 108 photons/pulse/harmonic at 42 eV in the experimental setup used in thisthesis.
One of the favorable properties of the high harmonic source with a hollow-core capillary is that the spatial mode of the generated harmonic beam is rather Gaussian in shape.
This allows experimenters to focus the harmonic beam into a small spot size for the investigation of physical phenomena on a microscopic scale (nm in principle), as will be discussed in Sec. 2.2.1. Another desirable property of the high harmonic beam is its spatial
and temporal coherence, as pointed out in the previous paragraph. Although UPS generally does not require a coherent source, coherence is a valuable property in many physicalinvestigations. Since pulses generated by synchrotrons are not coherent, high harmonic
generation light is not only a convenient replacement of the synchrotron radiation, but
unique properties, such as coherence and sub-femtosecond pulse width.

39

2.2 Time-resolved extreme ultraviolet photoemission spectrometer
The basic method of obtaining time-resolved information of physical phenomena iswell understood and has been implemented in a variety of physical investigations. One
basic method used to measure physical transients is the “pump-probe” technique. A laser
pulse is introduced into the system, which starts a physical process. Another laser pulse
is then sent into the system after some time-delay ∆t with respect to the first pulse. Thissecond pulse “probes” the status of the system of that moment. Various signals can bemeasured, including changes of the material reflection and absorption. After probing thesystem at several time delays and comparing changes among the spectra, a dynamic pictureof the physical process is mapped out. Although the idea of pump-probe spectroscopy issimple, measuring the dynamics of surface chemical reactions had not been performed
until now. This can now be done by combining two different technologies (UPS and
HHG) into one.
The “Time-Resolved extreme Ultraviolet Photoemission Spectrometer” (TR-UPS) that
I have constructed is shown in Fig. 2.4. The system consists of two major components –an ultra-high vacuum (UHV) chamber and a pair of IR pump/HHG probe beam lines.
2.2.1 Pump - HHG Probe beam line
As shown in Fig. 2.4, an amplified Ti:sapphire laser system is used as the light sourcein this experiment. The laser can generate a laser fluence of 1.5 mJ/pulse at a 2 kHz
repetition rate. The pulse width is measured to be 22 fs by frequency resolved opticalgating (FROG) [54]. The laser beam is directed to a beam splitter, which separates thelaser beam into two different pulses – 80% of the laser beam energy will be used as theSputter Gun

1000 l/s

Photo Diode

r
ge
ED
/A
u

LE

Ti:sapphire Laser System
1.5 mJ/pulse @ 2 KHz, 22 fs

Rough

Argon

Rough

70 l/s

EUV Multilayer
Reflector Pair

Delay Stage

Gate
Valve

WindowAl
FilterEUV

Gas Doser

Low Lux Camera

TOF

Pt(111)(SHG and P screen
mounted beneath)RGA

IR Filter

40

Figure 2.4: Schematic drawing of the time-resolved ultraviolet photoemission spectroscope. The pump beam is the beam line drawn at the lower part of the figure;the probe beam is drawn at the upper part of the figure, and it goes throughthe hollow-core capillary to generate high harmonics. The main chamber isunder ultra-high vacuum condition at all times (∼ 10−10 torr), and a gate valve
is used to separate the main chamber from the rest of the system. The multilayer mirror chamber and the beam line are under high vacuum (∼ 10 −6 torr)
condition.

41
probe, and 20% of the energy becomes the pump. The probe energy will be used for thehigh harmonic generation process to convert photons in the extreme ultraviolet, and theenergy of the resulting EUV pulses will be much less then the energy of the pump beam.
The time delay between the pump and probe beams is varied by using a translation stage in the pump beam. A micro-stepping motor with nanometer step resolution
is mounted to the translation stage to give a time delay resolution of better than 1 fs. 2 Thepump beam is then loosely focused into the UHV chamber and subsequently redirected tothe sample. The target position of the pump beam on the sample can be readily adjusted
by tweaking the mirror mounts outside the vacuum chamber.
For the probe beam, a hollow-core capillary is used to generate high harmonics. Argon gas is used as the nonlinear media for all the experiments discussed in this thesis. The
targeted harmonic used in these experiments is the 27th harmonic of the driving 800 nm
ultrafast pulses (42 eV in photon energy or 30 nm in optical wavelength). As discussed
in Sec. 2.1, harmonic generation produces a semi-continuum spectrum, so frequency discrimination is needed in order to be used in UPS. Traditionally, curved gratings are used
for this purpose; however, this scheme suffers from the fact that the reflectivity of thegrating of this wavelength is extremely low and the grating itself is difficult and expensive to fabricate. To make matters worse, in order to maintain the sub-picosecond pulse
width of the harmonic, two gratings are needed to form a zero-dispersion imaging system[55, 56]. This further reduces the already low reflectivity of the grating spectrometer. In
order to avoid this problem, a pair of multi-layer mirrors is used in our setup. This pair ofmulti-layer mirrors is made by periodically growing silicon and molybdenum thin-films
2

The translational stage moves 150 nm to increase or delay 1 fs.

42
on a super-polished glass substrate to form a Bragg reflector for the incoming EUV photons. The wavelength with the maximum reflectivity is equal to twice the thickness of the
Si/Mo stack of the multi-layer mirrors, and the combined reflectivity of the two mirrors
is shown in Fig. 2.5. The wavelength of the maximum reflectivity of the multi-layer mirrors is near the 27th harmonic, with a little less than 4% total reflectivity. The adjacentharmonics (25th and 29th ) save only 0.5% reflectivity, which effectively filter out theseharmonics. The rear multi-layer mirror has a radius of curvature of one meter; hence themulti-layer mirrors also have the benefit of focusing the 27th harmonic onto a small spotof the sample. Fig. 2.6 shows the image of the focal spot of the 27 th harmonic. This image
is taken by placing a phosphor screen at the sample position, then recording the phosphor
fluorescence. The spot size is measured to be 400µm with a Gaussian profile. The spot
size is large because of the long focal length of the curved multi-layer mirror used, andbecause the sample is not placed exactly at the focal position for convenience. Despite
this, the EUV spot size is still small enough for use in photoemission experiments. The
infrared pump beam spot size is arranged to be several times bigger than the EUV spot
size, to ensure that the probing region of the sample is uniformly illuminated.
A 200 nm thick aluminum filter is inserted between the UHV chamber and the beamline. The transmission of this filter at 42 eV is calculated to be 63% [57]. However, due tothe formation of an oxide layer on the surface, the transmission is more typically ∼ 10%.
The aluminum filter is retracted when the experiment is not running, and a gate valve isshut to separate the beam line from the UHV chamber. The aluminum filter serves two
purposes. Firstly, the driving 800 nm pulse is completely reflected by the filter, while the

43
4.5

27th (42eV)

4.0
3.5

Reflectivity (%)

3.0
2.5
2.0

29th (45eV)

1.5

25th (39eV)

1.0
0.5
0.0
26

27

28

29

30

31

32

33

Wavelength (nm)Figure 2.5: Combined reflectivity of the Si/Mo multi-layer mirror pair (2 mirrors) used in
the TR-UPS setup. The reflectivity of the mirrors are measured by the synchrotron radiation facility in Lawrence Berkeley laboratory.

1mm
Figure 2.6: The image of the 27th harmonic fluoresce at a phosphor screen. The phosphor
screen is mounted beneath the platinum sample, and the phosphor screen was
raised to the sample position when this picture was taken. The image was capture by a low-lux, high sensitive camera mounted outside the UHV chamberand looking in through a view port. The spot size of the harmonic is measured
to be 400 µm. The spot seems elliptical because the phosphor screen is at 45
degrees with respect to the incoming EUV beam.

44
harmonics pass through and enter the main chamber. Secondly, the filter is mounted on an
air-tight gate valve, which maintains the ultra-high vacuum in the main chamber.
The number of photons that reaches the sample can be calculated using the overall
transmission: 108 × 4% × 10% ≈ 4 × 105 photons for the 27th harmonic, with about a
1 eV energy bandwidth.
2.2.2 Ultra-high Vacuum Chamber
The UHV chamber is pumped by a 500 l/s turbo pump (Balzers TMU-520-SG) and a
270 l/s ion pump (Varian VacIon Plus 300 noble diode) to achieve a base vacuum pressureof under 1 × 10−10 torr after a standard UHV bake out. To prevent the earth’s magnetic
field from penetrating into the vacuum chamber, the chamber is shielded internally by double layers of µ-metal. This prevents the course of photoelectrons from being distorted byexternal magnetic fields. A platinum(111) single crystal is mounted inside the chamber by
spot-welding it onto two parallel tantalum heating wires, which are in turn spot-welded toa manipulator mounted vertically from the top port of the chamber. The manipulator has
the capability of performing three-axis translational and azimuthal rotational motion withrespect to the platinum sample. In addition, the manipulator’s inner tube can be filled withliquid nitrogen, providing cooling of the sample. A high current power supply (HP 6269B)
is connected to the manipulator and drives the tantalum wires that heat the platinum sample. A proportional-integral-derivative (PID) temperature controller (Omega CN77000)
controls this power supply. A K-type thermalcouple is also spot-welded to the back of
the platinum sample for measuring the sample’s instantaneous temperature. The measured
temperature feeds into the PID temperature controller as a feedback signal to control the45high current power supply. Using this feedback control scheme, the electrical heating canbe nicely balanced by the liquid nitrogen cooling; therefore, the sample temperature can bereliably controlled over a wide temperature range, from 77 to 1200 K, and the temperature
can be increased or decreased at a rapid rate.
A custom-built time-of-flight (TOF) electronic kinetic energy analyzer is used for detection. Two shielding layers of µ-metal are also installed around the inner wall of the TOF
tube to insulate it from the earth’s magnetic field. The total tube length is L tube = 600 mm
and the inner diameter is 40 mm, which provides an electron collection solid angle ofS ' 2 × 10−3 sterradians. At the end of the TOF tube, a custom-built large area (40 mm)
fast-response detector detects the electrons. A double micro-channel plate (MCP) is usedat the front of the detector to obtain the gain necessary to detect single electrons. The
MCPs amplify the single electron signal by a secondary electron cascade process to detectable signal levels (∼ 100 mV). The detector has timing resolution of about 200 ps.
Impedance matching of the whole electron detector allows it to operate with a GHz frequency bandwidth. The key ideas of impedance matching are adapted from the designof A. Stolow [58], and a cone-shape anode design ensures that the fast electronic signal
propagates into the coaxial cable with no noticeable reflection. Figure 2.7 shows the output signal when an electron arrives at the detector. The signal has a rise time of less than
1 ns and a pulse width of less than 2 ns. The signal is clean, without noticeable reflection. This is crucial to the detection of multiple incoming electrons. The output signal issubsequently delivered to a high frequency pre-amplifier followed by a constant fraction
discriminator (EG&G ORTEC 9327), ensuring that the electron arrival time is independent

46
of output signal intensity fluctuations. An all solid-state time-to-digital converter (TDC)(ACAM AM-F1) measures the electron flight times, referenced to a start signal from theultrafast laser system. The real electron flight time is actually referenced to the moment
when the EUV pulse arrives at the sample, also the moment when the sample starts to emit
electrons. This moment appears on the TOF spectrum as an EUV scattered light peak.
This peak is a result of the EUV pulse being scattered/reflected from the sample surface,with the scattered EUV light then triggering the detector. Actually, there is a 2 ns timedelay between this EUV peak and the real start time, resulting from the propagation of the
EUV light to travel from the sample to the detector. This time difference is neglectable
due to the fastest photoelectron needs to take more than 100 ns to reach the detector. By
subtracting the time of the EUV “start peak” from the measured flight time, the actual
electron kinetic energy Ekin can be calculated by the following equation1
Ltube 2
Ekin = me (
)
2
tf

(2.7)

where me is the electron mass, tf is the subtracted electron flight time.
If the electron emission is assumed to be evenly distributed over the 2π solid angle(half sphere) for rough estimation, the count rate of photoelectron detection C R (electrons/second) can be calculated using
CR =S(1 − R)I · α · D · Lrep
2π

(2.8)

where S = 2 × 10−3 is the solid angle of the TOF, I = 4 × 105 is the incoming photon
number per pulse, R is the reflectivity of the sample, and Lrep = 2 kHz is the ultrafast laser
repetition rate. α ' 8% is the photoelectric yield of platinum surface at 30 nm [59, 60]. D470Singal (mV)

-20

-40

-60

-80
-30

-20

-10

0

10

20

30

Time (ns)Figure 2.7: The output signal of the impedance matched MCP TOF detector when a single
electron hits the detector. The signal is directly measured from the detector
with an oscilloscope. It is very important that there is no signal reflection or“ringing” after the main peak for high count rate electron detection.
is the detection efficiency for the MCP detector, and is measured to be ∼ 50 − 85% for theelectron kinetic energy range of 0.2 − 2 keV [61]. For the platinum (111) single crystal, at
an incident angle of 45 degrees, R = 9% [57]. Therefore, the estimated count rate is C R ≈1.5 × 104 electrons/second, while the experimental count rate is also CRexp ≈ 1.5 × 104
electrons/second.
Figure 2.8 shows the inside of the UHV chamber. For standard surface diagnostics, theUHV chamber is equipped with a Low Energy Electron Diffraction (LEED) Spectrometer48
and an Auger Electron Spectrometer (AES) (OCI BDL800IR), which monitor the sample
surface’s condition. The diffraction pattern of the LEED can reveal the local order ofthe sample surface, while surface contamination can be seen using the AES. A residual
gas analyzer (RGA) (Leybold Inficon Transpector) is installed on the UHV chamber formonitoring the ambient chamber gas content. In addition, Thermal Desorption Spectra(TDS) can be taken by the RGA, measuring the partial pressure of a target molecule while
the temperature of the sample is steadily raised. A TDS measurement for the O 2 /Pt(111)system is shown in Fig. 3.2.
For sample preparation and cleaning, an argon ion sputter gun is installed in the UHV
main chamber. The sputter gun can be used to remove the topmost several mono-layers of
the platinum sample; sample surface contaminations are simultaneously removed. Rapid
thermal annealing can subsequently be performed to reconstruct the surface order.
Finally, a custom-built gas doser is used for precision gas dosing. The major benefit ofusing a gas doser versus using a gas leak valve is that a doser allows a much higher local
pressure to be created around the sample area while keeping the rest of the chamber inhigh vacuum. This also minimizes the opportunity for gas molecules to adsorb the UHV
main chamber wall, preventing prolonged pump down times after gas dosing. In addition,
if experimentally required, continuous gas dosing to the sample would not cause serious
problems. Maintaining the high vacuum condition of the rest of the system is needed toensure the functionality of other highly sensitive equipment. For example, the working
pressure of the MCP electronic detector must be lower than 1 × 10−6 torr.

49

Figure 2.8: Picture shows the internal arrangement of the TR-UPS chamber. In this photo,
the sample is turned 90 degrees from the normal position for better viewing.
Both of the pump-probe beams are coming in from the flange at the left sideof the picture to reach the metal sample.

50
2.2.3 Finding temporal and spatial overlap
For any pump-probe experiment to be successful, finding the temporal and spatial overlap between the pump and probe beams is critical. In this experiment, the relatively low
intensity of the high harmonic generated EUV beam creates a particular alignment challenge. In addition, it is a challenge to find the temporal overlap between an EUV (probe)
pulse and an infrared (pump) pulse [62]. Therefore, to routinely run this experiment, aneffective method for finding both the temporal and spatial overlap is needed.
To find the temporal overlap, or so-called “time-zero”, some of the infrared light used
to create the EUV probe beam is allowed to enter the EUV chamber, by retracting thealuminum filter and inserting a thin sapphire window instead. The argon gas of the highharmonic cell is turned off to reduce the pressure in the beam line. A KDP (second
harmonic) crystal mounted underneath the platinum sample is now raised to the sample
position, and the time-zero can then be found by looking for the second harmonic crosscorrelation signal between the pump and the probe pulses. Since the aluminum filter isonly 200 nm thick, the time difference between the generated EUV pulse and the driving
infrared pulse reaching the platinum sample is negligible. The time difference due to theremoval of the argon gas is also negligible because of the low pressure (∼ 50 torr) andshort length (8 cm) of the gas interaction region. However, the time difference induced byinserting a sapphire window to the infrared pulse needs to be accounted for. The thicknessof the sapphire window is 355 µm, hence the time delay induced by the sapphire can be
precisely calculated to be 934 fs. This time difference can be corrected for by moving thetranslational stage after the cross-correlation is found. Therefore, it is effective to find the

51

4.0

SHG Intensity (arb. unit)

3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-150 -100 -50

050 100 150

Time Delay (fs)Figure 2.9: Signal of second harmonic generation between the infrared pump and probe
pulses. The FWHM of the signal is 62 fs.
time-zero between the probe and pump beam without directly using the EUV pulses. A
typical cross-correlation signal for the two beams is shown in Fig. 2.9.
To find the spatial overlap, a phosphor screen (P1 phosphor) mounted beneath thesample is raised to the sample position. The focused EUV spot can be imaged by a highly
sensitive, low-lux CCD camera (Minitron 12V1E) mounted outside the chamber. The
camera views the phosphorescence through a view port, as shown in Fig. 2.6. Since the52beam pointing of the EUV is fixed and cannot be adjusted, spatial overlap is achieved bymoving the infrared pump beam location by adjusting the mirror mounts outside of thechamber. Once the spatial overlap is found, the sample is lowered back to its original
position.

2.3 Conclusion
In this chapter, the implementation of the TR-UPS setup was thoroughly discussed
and explained. In the following two chapters, the setup will be utilized to measure chemical dynamics of adsorbates on metal surfaces, demonstrating the power of the TR-UPS
technique.

CHAPTER III

Probing the transition dynamics of chemisorbed states of
O2 on Pt(111)

3.1 O2 states on Pt(111) surfaces
Because of its importance and relative simplicity, the O2 /Pt(111) system is considered
to be the model system for catalytic chemistry, and has been investigated both experimentally and theoretically in considerable detail. Four different adsorption states of oxygenon Pt(111) have been reported to date: one physisorbed state, two chemisorbed states, andone atomic state. It is important to review some of their properties before going further oninvestigating dynamics of oxygen molecules on Pt(111).
3.1.1 Physisorbed State
The physisorbed state of molecular oxygen can be stably deposited on a platinum surface when the surface temperature is below 30 K. In the case of physisorbtion, chargetransfer or bond hybridizaton do not occur between the oxygen molecular bonds and theplatinum d-band. The force holding the oxygen molecule to the platinum surface is a vander Waals attraction; hence, the physisorbed oxygen molecules do not reside on any specific sites on the platinum surface. Multiple layers of physisorbed oxygen molecules can

53

54

hcp sitefcc siteSuperoxo oxygen(bridge site)

Peroxo oxygen(hollow site)

Atomic oxygen(hollow site)

Figure 3.1: Schematic diagram of the surface configuration of oxygen on Pt (111), for the2−
superoxo (O−2 ), peroxo (O2 ), and atomic (O) oxygen states.
reside on the metal surface, and these layers can be considered as a “condensed” phase ofmolecular oxygen. The electronic properties of these molecules are very similar to thoseof the gas-phase free oxygen molecules.
3.1.2 Superoxo and peroxo chemisorbed states
There are two different chemisorbed oxygen molecular states for the platinum (111)2−
surface: superoxo oxygen (O−2 ), and peroxo oxygen (O2 ). When oxygen molecules aredeposited on a platinum (111) surface at a temperature of about 77 K, thermal agitationof the oxygen molecules is stronger than the van der Waals attraction, and oxygen can
no longer be physisorbed on the platinum surface. Instead, oxygen molecules attach tothe platinum surface by chemisorption. Current wisdom is that during the chemisorption
process, electrons transfer from the the platinum d-band to the molecular 1π g∗ orbital of
the oxygen molecules, and the oxygen molecules become negatively charged. Therefore,

55
the electronic configuration of the oxygen changes considerably, especially for molecularorbitals with low binding energies.
“Superoxo” oxygen resides on a bridge site between two platinum surface atoms, with
the intermolecular axis between the two oxygen atoms parallel to the surface (perpendicular to the surface normal). The O-O bond length of the superoxo state is calculated tobe 1.39 Å using density functional theory, which is in good agreement with experimental measurements [63, 64]. The adsorption energy of the superoxo molecular oxygen isestimated experimentally to be -0.4 eV/molecule [65]. The adsorption bond between the
superoxo oxygen and the platinum surface is formed by one electron transferred from theplatinum surface to the now half-filled oxygen 1πg∗ orbital.
The “peroxo” chemisorbed state resides on the fcc three-fold-hollow site of the platinum surface. The intermolecular axis also nearly parallels the surface, but with a slightly
tilted angle. The O-O bond length for the peroxo state is calculated to be 1.43 Å, andin agreement with experimental estimates [63, 66]. The adsorption energy of the peroxo
molecular oxygen is experimentally measured to be -0.5 eV/molecule [67]. The adsorption bond between the peroxo oxygen and the platinum surface is formed by two electronstransferred to the molecule, resulting in a fully filled oxygen 1πg∗ orbital. The adsorption energies and bond length information for the different oxygen states on the platinum
surfaces are summarized in Table 3.1.
Several properties of these two forms of chemisorbed molecular oxygen states differ.
The peroxo oxygen is slightly more tightly bound to the platinum surface than superoxo.
In the peroxo species, one more electron is transferred from the platinum surface to the56

Gas-phase oxygen molecules (O2 )Superoxo molecular oxygen (O−2)
2−Peroxo molecular oxygen (O2 )Atomic oxygen (O) (fcc)
Atomic oxygen (O) (hcp)

Adsorption Energy(eV/molecule)
—
-0.40
-0.50
-1.65
-0.98

Bond Length
(Å)
1.21
1.39
1.43
—
—

Table 3.1: Comparisons of the adsorption energies and bond lengths between gas-phase2−oxygen molecules (O2 ), superoxo (O−2 ), peroxo molecular oxygen (O2 ), andatomic oxygen (O) on fcc and hcp sites.
1πg∗ orbital, forming two hybridized bonds between the peroxo oxygen molecule and theplatinum surface. Increasing charge occupation in the 1πg∗ orbital strengthens the connection between the oxygen molecule and the platinum surface; the 1π g∗ orbital of theoxygen molecule is, however, an anti-bonding orbital with respect to the oxygen intermolecular bond. Therefore, the intermolecular bond connecting the two oxygen atoms is
weaker when the oxygen is adsorbed on the peroxo state as opposed to the superoxo state;
this property can be confirmed by the fact that the bond length of the peroxo oxygen isslightly longer than that of the superoxo oxygen. In fact, both of the oxygen molecularchemisorbed states (superoxo and peroxo) have a longer bond length (1.39 Å and 1.42 Å)compared to the free oxygen molecules (1.21 Å). Because of this, the two chemisorbed
molecular oxygen states are often called the “intermediate” or “precursor” states of chemical oxidation processes on metal surfaces. For many chemical oxidation processes, dissociation of oxygen molecules into two atomic oxygens is necessary to initiate the process.
It is suggested that the oxygen dissociation progresses gradually through weakening of theoxygen intermolecular bond (by transferring the oxygen molecules to these intermediate
states) and finally to complete dissociation [68, 69, 70].

57
Another difference between these two chemisorbed states is that the superoxo oxygen tends to cluster on the adsorption sites, whereas the peroxo oxygen tends to be isolated. This property has been confirmed by observing the states through the technique ofscanning tunneling microscopy (STM) [71]. This could indicate that intermolecular interactions, either attractive or repulsive, between the adsorbates can be a crucial factor instabilizing these two chemisorbed states on the platinum surface. The clustering effect of
the superoxo oxygen can explain why, at the same surface temperature, superoxo oxygen
tends to dominate with high oxygen coverage and peroxo oxygen tends to dominate withlow oxygen coverage. In addition, observation from STM measurement shows that boththe chemisorbed states are stable, and diffusion of chemisorbed molecules after adsorption
is not observed. This stability also indicates that superoxo oxygen clustering is not due todiffusion of chemisorbed molecules, but to the existence of a mobile precursor state uponadsorption [71].
STM work has also demonstrated that it is possible to use the STM tip to manipulate anoxygen molecule, moving it between these two chemisorbed states (superoxo and peroxo)
from the bridge to hollow site, or vice versa [72]. The STM tip can be positioned directly
above the oxygen molecule either in the bridge or the hollow site. After applying a verysmall current (nA) to the STM tip, the oxygen molecule gains energy to overcome thebarrier of the potential energy surface, and finally stably resides on another adsorption site
without perturbing other neighboring oxygen molecules.

58
3.1.3 Atomic Oxygen
Atomic oxygen can be formed on Pt(111) by dosing oxygen molecules at above 150 K
surface temperature. At this temperature the oxygen molecules dissociate before becoming
adsorbed to the platinum surface. Atomic oxygen can also be formed by dosing oxygen
molecules at lower surface temperatures and subsequently heating the platinum surface toabove 150 K for a short period of time to dissociate adsorbed oxygen molecules [73, 74].
Oxygen atoms are energetically favorable to reside at either the fcc or the hcp threefold
hollow sites (fcc sites have no platinum atom directly beneath in the second surface layer)[71]. Theoretical calculations have estimated that the adsorption energy is much larger
in the fcc hollow sites (E = −1.65 eV/molecule) and considerably smaller in the hcp
hollows (E = −0.98 eV/molecule) [70].

3.2 Experimental results of O2 /Pt (111)3.2.1 Thermal Desorption Spectroscopy
Thermal desorption spectroscopy (TDS) 1 is a well-known and commonly used technique for characterizing chemical reaction processes on surfaces. The idea of TDS is toadsorb the molecules under study to the metal surface at a low non-reactive temperature.
The temperature of the metal surface subsequently increases at a constant rate to a temperature well above the temperature where the adsorbates react and desorb. By measuring theyield of the desorbing molecular species and the corresponding desorbing temperatures,information about adsorption energy of the adsorbates, activation energy, and the reaction
channels of the chemical reactions can be extracted. For oxygen on platinum (111), TDS
1Some literature refers to temperature programmed desorption spectroscopy

O2 Desorption Signal

59

100

TDS
O2/Pt(111)
α-O2

β-O2

200

600

x5

800

1000

Temperature(K)Figure 3.2: TDS for Pt(111) covered with O2 : the rate of temperature increase is 3 K/sec
from 77 K to 300 K; then 10 K/sec from 300 K to 1100 K
can help us understand the oxygen chemisorbed states and their corresponding properties.
Figure 3.2 shows the TDS spectrum of O−
2 on Pt (111) taken using our setup. Oxygen
was initially dosed onto the platinum sample at 77 K, to saturation coverage. Two distinct
desorption peaks can be observed in the TDS spectrum: An intense α − O 2 peak at 145 K,
and a weaker high temperature β − O2 peak (the symbols of α and β are used for identification purposes only; the oxygen molecules desorbing from these two peaks are the same).
A reasonable explanation for the TDS spectrum of O2 /Pt is as follows: As discussed in

60
the previous section, superoxo oxygen (O−2 ) dominates at the bridge site at high coveragewhen oxygen is dosed to full coverage at liquid nitrogen temperature. When the surface
temperature is heated above 145 K, a large fraction of oxygen molecules desorbs from theplatinum surface, since the oxygen molecules have gained enough energy to overcome theenergy barrier for desorption. Once some of the oxygen molecules desorb, the oxygen coverage becomes much lower on the platinum surface, and there will not be enough oxygen
molecules left on the surface to cluster at adjacent bridge sites. The oxygen molecules now
tend to move from the bridge site to the three-fold-hollow sites to lower their total energy,
2−leading to a state transition from the superoxo (O−2 ) into the peroxo (O2 ) state for theremaining oxygen molecules. When the surface temperature increases above 200 K, theintermolecular bond of the adsorbed oxygen molecule will gain enough energy to be broken. This causes the oxygen molecule to dissociate into two identical oxygen atoms (O),and the two oxygen atoms absorb on two different three-fold-hollow sites on a platinum
surface. Since the atomic oxygens have a much higher adsorption energy, they will remainon the platinum surface until the surface temperature reaches temperatures higher than800 K. Finally, the atomic oxygen will desorb recombinatively to form oxygen molecules
and leave the platinum surface.
3.2.2 Static photoemission spectra of the different oxygen states
From the knowledge gained from TDS investigation, we can now selectively prepare
the oxygen adsorbed on the platinum surface preferentially for a certain oxygen state. Experimentally, saturation coverage leads to superoxo oxygen as dominant state. Therfore, toprepare superoxo oxygen, oxygen can simply be dosed onto a platinum surface to the sat-

61
uration coverage at liquid nitrogen temperature, so that most of the oxygen molecules willcluster and reside on the bridge sides of the platinum surface. By heating the platinum
surface to 145 K, a partial desorption of molecular oxygen increases the occupation of
three-fold-hollow sites for the oxygen molecules, and leads to a predominance of the peroxo chemisorbed state. At temperatures above 145 K, oxygen molecules will gain enoughenergy to dissociate into oxygen atoms; therefore, heating the sample to room temperatureis the preferable way of obtaining atomic oxygen [14, 63, 65, 69, 72, 75].
Static photoemission spectra have been taken using our TR-UPS setup by blocking the2−pump pulses to avoid excitation for the clean Pt(111) surface, O −2 /Pt(111), O2 /Pt(111),

and O/Pt(111). Comparing the spectrum of the clean Pt(111) surface to the other oxygen
covered spectra reveals that the intensity of the platinum d-band, where the binding energy
is close to the Fermi-edge, is strongly reduced for the latter spectra. This surface sensitivity is expected – when oxygen molecules adsorb on the platinum surface, the electrons
emitting from the platinum are scattered back into the platinum due to the presence of theoxygen molecules.
At a binding energy of about 6 eV (referenced to the Fermi level), an oxygen-related
feature appears in the oxygen covered spectra. This feature has been observed previously
by others using synchrotron radiation sources, where it has been assigned to the occupied oxygen 1πg∗ orbitals of the free oxygen molecule [66]. The position of this featureappearing at about 6 eV binding energy agrees with calculations of density functional theory [70, 76] that the oxygen molecule absorbed at the three-fold-hollow site has a higher
density-of-states at this energy. At the EUV photon energy of 42 eV used in our setup, thisPhotoemission Yield (arb unit)

62

O

O22

-

O2
9

7

5

3

Binding Energy (eV)2−
Figure 3.3: Static photoemission spectra taken for the superoxo (O −2 ), peroxo (O2 ) andatomic (O) oxygen states on Pt(111).

63
feature is particularly sensitive to the chemical bond character of the oxygen chemisorbed
configuration. Close-up photoemission spectra are shown in Fig. 3.3 for the superoxo
2−(O−2 ), peroxo (O2 ) and atomic (O) oxygen states on Pt (111) around the binding energy

from 3 to 9 eV. In qualitative agreement with results by Puglia and co-workers [63], we
observe a change in the spectral shape of the 1πg∗ orbital between the superoxo and peroxo
states, but no significant difference in this energy region between the peroxo oxygen andthe atomic oxygen states. This change is due to the different degree of hybridizaton of
the particular oxygen state to the platinum d-band. Comparatively, the spectrum of thesuperoxo oxygen remains relatively flat in this energy region, similar to the clean platinum
surface, while the spectrum of peroxo and atomic oxygen has a “peak” at about 6 eV binding energy. The spectra of peroxo oxygen and atomic oxygen have similar features in thisregion because the peroxo oxygen resides in a geometry similar to the atomic oxygen, andalso the peroxo oxygen is considered to be the “precursor” state in the process of moleculardissociation. In the following experiment to probe the oxygen dynamics, this peak will be
used as a valuable “signature” to indicate whether the oxygen is changing its chemisorbed
configuration, by observing the change of this peak after the O 2 /Pt system is triggered byan intense ultrafast pulse.
3.2.3 Oxygen desorption induced by intense ultrafast pulses
Desorption by intense femtosecond laser pulses of O2 /Pt(111) has been studied previously by photo-desorption yield measurements [14, 77]. In particular, a nonlinear dependence of desorption yield with increasing laser intensity has been reported. In fact, thisnonlinear dependence has been reported for many adsorbate/metal systems and proposed

64
as the result of direct coupling between excited hot surface electrons and adsorbates, without conventional heating from the surface lattice, or phonons [78]. In Mazur’s desorption
work [77], he proposed that after the sample was excited by an ultrafast pulse, a new “configuration” for the adsorbed oxygen molecules is created. The oxygen molecules could
settle in a configuration that differs from a “fresh” platinum surface. This new configuration might complicate further subsequent laser desorption and deposition cycles resultin a decreasing desorption yield. Figure 3.4 shows femtosecond-induced desorption yield
traces for two adjacent desorption cycles with oxygen redosing in between. This observation indicates that a surface modification other than complete oxygen depletion occurs,
resulting in a diminished number of free adsorption sites in subsequent desorption cycles.
One reasonable explanation is that the high intensity laser pulse not only desorbs the oxygen from the platinum surface, but the oxygen molecules are also dissociated into oxygen
atoms. These oxygen atoms then block the adsorption site. In general, nonreversible effects, such as dissociation, are not desirable for time-resolved photoemission experiments
and should be minimized. Each photoemission spectrum at each time delay needs to beaccumulated through many pump-probe excitations. Hence, a large advantage is realized
if the system under study reverts to its original state before another excitation cycle starts.
Otherwise, reversible and irreversible changes are superimposed making data interpretation difficult. Therefore, in all of our experiments, we used a low (10-15 µJ/mm 2 ) pumpfluence to minimize nonreversible processes, such as desorption and dissociation. Fromextrapolation of control experimental data, we estimate that significant irrversible oxygen
desorption requires a time scale of more than an hour for the pump excitation levels used65
in our time-resolved work.
3.2.4 Probing O2 /Pt(111) by TR-UPS
We start by adsorbing a saturation layer of molecular oxygen onto the platinum surface
at liquid nitrogen temperature (77 K). Photoemission spectra were taken at different timedelays between the pump infrared pulse and the EUV probe pulse. Significant changes
were observed in the spectra as a function of time delay between the pump and the probe in
two different energy regions: at the Fermi edge, and at a binding energy of 6 eV reference
to the Fermi level, where the oxygen-induced feature (1πg∗ ) appears.
Figure 3.5 shows the transient modifications at the Fermi edge as observed at different
delays. At time zero (i.e., when the pump and probe pulses arrive temporally coincident
at the surface), the shape of the Fermi edge is modified by a “step” at energies above theFermi edge with a width of about 1.5 eV, corresponding to the energy of the exciting photons. Some evidence of this feature is still visible at 150 fs delay. Within another 100 fs,
or 250 fs after the pulse pulse arrives, this nonthermal contribution has completely disappeared. At longer time delays, no further changes in this energy region were observed. The
experimental distribution at time-zero is best reproduced by an excitation level of about
6% (at a photon energy of 1.5 eV). Referring to the insert of Fig. 3.5, the solid line isthe calculated distribution for this excitation level, convolved with the estimated detector
resolution of 370 meV. For reference, a spectrum taken with solely with the probe EUV
beam (ground-state distribution) is fitted to a Fermi-Dirac distribution at 100 K, which isalso convolved with the detector resolution.
Figure 3.6(b-e) shows the pump-probe spectra at various time delays, focusing on

66

O2 Desorption (arb. unit)

integrated desorption signal

1.0

first cycle

0.5

second cycle

0.0
0

50

100

time (sec)

1st desorption cycle
2nd desorption cycle

0

50

100

Time (sec)Figure 3.4: Femtosecond laser-induced desorption for the two adjacent desorption cycles
with oxygen redosing between the two laser excitations. Insert shows the integrated desorption signal for the two cycles.

Photoemission Yield (arb. unit)

Photoemission Yield (arb. unit)

67

1

time-zero
without pump

0,1

0, 1
0

-1

-2

Binding Energy (eV)

0 fs
150 fs
250 fs
2 ps

-1

-2

-3

Binding Energy (eV)Figure 3.5: Electron energy distribution at Fermi edge of O2 /Pt(111), for time delays of
0 fs, 150 fs, 250 fs, and 2 ps. Nonequilibrium hot electrons can be seen at 0
and 150 fs; however, at 250 fs, the energy distribution has thermalized. Inset: Fermi edges at 0 fs time delay (black square) and static spectrum without
pump pulses excitation (hollow square), plotted in log scale. The electron distribution at 0 fs time delay can be fit to a 100 K Fermi-Dirac distribution with
6% of the electrons excited by the pump photon energy (1.5 eV).

68(b) 0fs(c) 250fs(d) 500fs(e) 0fs

Counts (arb. unit)(a) no pump

10

7

4 10

7

4 10 7 4 10 7
Binding energy (eV)

4 10

7

4

Figure 3.6: Time-resolved high-resolution photoemission spectra from a saturation layerof molecular oxygen adsorbed on a Pt(111) surface at liquid nitrogen temperature: (a) no pump beam; (b) with pump beam, zero time delay between pumpand probe; (c) 250 fs time delay between pump and probe; (d) 500 fs timedelay; (e) repeat of (b), taken immediately after (d).
the higher binding energy part of the valence band spectra. For comparison purpose, aphoto-electron spectrum without pump beam excitation was taken in the very beginning,as shown in Fig. 3.6(a). In these spectra, the feature appears at around 6 eV binding energy
corresponding to the oxygen 1πg∗ orbital hybrized with the platinum surface, as discussed
in previous paragraphs. Figure 3.6(b) shows the spectrum obtained at zero time delay; no
significant changes are visible when compared to the static spectrum. This contrasts withthe changes above the Fermi-edge at zero time delay, as shown in Fig. 3.5, where the nonthermal distribution happens at zero time delay. However, at 250 fs time delay – well after
the 55 fs pump pulse has been adsorbed and the nonthermal distribution at the Fermi edge
has disappeared – a significant peak-like structure emerges out of the background. This
peak-like structure is even stronger at a pump-probe delay of 500 fs, and it persists for69

Integrated Peak Intensity (arb unit)

2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Time Delay (ps)Figure 3.7: Integrated amplitude of the characteristic 6 eV peak feature as a function of
pump-probe delay. A fit to the data indicates an onset time of 550 ± 140 fs.
several ps. Although our spectra were taken sequentially in this experiment with increasing time delays, this observed feature is not a permanent change of the sample throughcontinuous pump pulse excitation, but rather a transient change. This can be verified bytaking another spectrum at zero time delay immediately after the 500 fs spectrum is taken.
This final time-zero spectrum is identical to the spectra of (a) and (b), which are taken
at the beginning of the pump-probe scans. Therefore, in this experiment, the observedchanges are definitely reversible, and a transient modification of the adsorbate bonding ismonitored with femtosecond timing resolution for the first time.
By repeating this experiment to acquire more detailed scans, we are able to extract

70
the characteristic population and relaxation rates of this transient (meta-stable) excitedadsorbate state. Figure 3.7 is a plot of the integrated strength of this transient peak as afunction of time delay. The data points can be fitted by an exponential growth function:y = y0 + Ae

(

x−x0
)
t0

(3.1)

where t0 is the rise time of the exponential increase function. By using this fitting equation
to the extracted data, we estimate that the observed peak has an exponential rise time
t0 = 550 ± 140 fs. Together with the information at longer time delays, we know that thesignal decays within about 5 ps after the pump pulse arrives.
3.2.5 Permanent modification of O2 on Pt(111)
In addition to the reversible change observed in the valence band structure, over thetime scale of several hours of continuous pump pulse excitation, we also observe a longterm nonreversible change of the surface state induced by the ultrafast infrared pumppulses. Figure 3.8 shows a series of spectra taken during a period of 2 hours of irradiationby the ultrafast infrared pump pulses. During the first hour of the pump pulse irradiation,
after a saturation coverage of oxygen was adsorbed to the platinum surface, no significant
changes in the spectrum are observed. However, further irradiation from the ultrafast infrared pulse pulses alters the 6 eV binding energy feature from a shoulder-like structure
to a peak-like structure. This peak-like structure is similar to the spectra of the peroxo
and atomic oxygen states on the platinum surface of their static spectrum; it is also similar
to the transient peak appearing in the pump-probe scans. This differs from the transientfeature, which disappears when the time delay returns to zero. By continuing ultrafast

71

17 mins

51 mins

102 mins

120 mins

Counts (arb. unit)

0 mins

7

2

7

2

7

2

7

2

7

2

Binding Energy (eV)Figure 3.8: Spectra taken during pump beam irradiation of the sample over a period of 2hours. During the first ≈ 50 min, the spectra are unchanged. However, after≈ 100 min, the characteristic 6 eV peak gradually increases and is permanently
present on the subsequent spectrum taken.
infrared pump pulses irradiation over several hours, this feature remains after the pumppulses have stopped, indicating the sample condition does not return to its original state.
This feature in the photoemission spectrum seems to represent a stable configuration of
the O2 /Pt(111) system, resulting from continued irradiation by the ultrafast infrared pumppulses over several hours.

3.3 Discussion
The reaction pathways of surface chemical reactions induced by an intense, ultrafast
infrared pulse could possibly proceed in following order:
1. Creation of a nonthermalized or “hot” electron distribution at the metal surface results from the energy adsorption of the excited ultrafast pulse.
2. Internal energy redistribution between the hot electrons to achieve a thermalized

72
configuration.
3. Adsorbate excitation caused by the energy transfer mediated by hot electrons fromthe metal surface.
4. Modification of the adsorbates-surface chemical bond character leads to translational and rotational motions of the adsorbates.
5. Thermalization between the electrons and the surface photons.
6. Traditional “phonon induced” chemical surface reaction caused by the elevated phonon
temperature, but this step happens in a much longer time scale (∼ps).

In the pump-probe experiment, the transient feature of the photoemission spectrum observed above the Fermi edge could be assigned to the step 1; while the transient peakobserved at the higher binding energy side of the valence band could be assigned to thestep 3. Furthermore, knowledge of the adsorbed fluence as well as the measured excitation
rate of the electron gas allow us to reconstruct steps 2, 4 and 5.
3.3.1 Nonthermal surface electrons on Pt(111) surface
3.3.1.1 Nonthermal surface electrons observed in the experiment
Laser energy adsorbed by the adsorbate layer (in this case, a saturated layer of molecular oxygen) is extremely low; therefore, direct excitation from the ultrafast infrared pumppulse is highly unlikely [78, 79]. The energy of the incident infrared pulse is adsorbed
entirely by the electrons on the top ∼ 100 Å thick of the platinum (111) surface. Becauseof the extremely short (∼ 65 fs) and intense (> 1012 W/mm2 ) excitation, this energy

73
results in a extremely nonthermal energy distribution of the electron gas. This nonthermal distribution can be observed at the spectrum taken at zero time delay, that shows atransient “step” above the Fermi-edge. The adsorbed fluence of 1 mJ/cm 2 results in an
estimated excitation of about 6% of the electrons at the platinum surface [80]. Comparable excitation levels have been reported for heating a 300 Å thick gold film [32] and aPt(110) surface [81] by ultrafast pulses. The fact that the transient nonthermal feature of
the “hot” electron step at the Fermi-edge disappears within the first 250 fs after the zerotime delay is in agreement with the measurement of the Pt(110) surface [81]. In contrast,
significantly longer lifetimes of the nonthermal contribution (up to 400 fs) have been reported for gold. This time difference between the platinum and the gold surfaces can be
explained as a much slower electron thermalization process in noble metals in comparison
to transition metals, because of a reduced electron-electron scattering rate at the relevantenergies [82, 83]. The measurement is extremely surface sensitive because the 42 eV EUV
probe photons we used can only liberate electrons from the top 5 Å of the platinum surface
due to the short electron penetration depth at these kinetic energies.
3.3.1.2 Two temperature model and nonthermal surface electrons lifetime
Knowledge of the absorbed fluence allows us to calculate the time evolution of theelectron and lattice temperatures. Due to its low heat capacity, the surface electrons areheated quite efficiently by ultrafast infrared pulses. Although the electron distribution immediately after the excitation is nonthermalized and technically cannot be fully described
quantitatively by a simple temperature, nevertheless the calculated electron temperature isa useful indicator of the actual excitation degree of the surface electrons.

74
Thermalization of the electron distribution occurs within 250 fs, followed by subsequent cooling of the hot electrons due to coupling to the surface lattice phonons (electronphonon coupling). The electron-phonon effective collision time is actually of the sameorder as the electron-electron scattering time. It is, however, quite inefficient with respect
to collisional energy exchange, because of the typical phonon energies required for the energy transition (about some tens of meV), where significant excitation of the surface latticeoccurs on a much longer time scale than the electron dynamics. In addition, the higher heat
capacity of the lattice limits the maximum lattice temperature to a much lower temperaturethan that of the electrons. On the other hand, since only the electrons at the top ∼ 100 Å
of the platinum surface are excited by the ultrafast pulses, thermal diffusion between theelectrons at the metal surface and the electrons at the bulk also has a significant effect on
the thermalization process.
The interplay between ultrafast pulses, hot electrons, and the lattice (phonons) cangenerally be calculated by a simplified model, or so-called “two-temperature model” [84].
A set of coupled equations is used to determine the electron temperature (T e ) and the lattice
temperature (Tp ) as a function of time:

∂
∂
∂
Te =
(κ Te ) − g(Te − Tp ) + Fb
∂t
∂z ∂z
∂
Cp Tp = g(Te − Tp )
∂t
Ce

(3.2)
(3.3)

where Ce = γTe and Cp are the electron and lattice heat capacities, respectively. κ =κ(Te /Tp ) is the thermal conductivity and g is the electron-phonon coupling constant. The

75
Electron specific heat γ
Thermal conductivity κ0 (77 K)
Electron-phonon coupling constant g
Lattice heat capacity Cp (77 K)
Absorbed laser peak intensity I0
Pulse width τ
Optical penetration depth λ (800nm)

748 J/m3 K2
71.6 W/mK
6.76 × 1017 W/m3 K
2.69 × 106 J/m3 K
1.71 × 1014 W/m2
55 fs
12.56 nmTable 3.2: Parameters of platinum(111) used in the calculation of electron and lattice temperatures
term F̂ is used to describe the femtosecond pump pulse in the form ofF̂ =I0 −4log(2)t/τ −z/λ
e
λ

(3.4)

where I0 is the adsorbed laser peak intensity, λ is the optical penetration depth, and τ isthe pump pulse width. All the parameters used in this simulation are listed in Table 3.2.
The calculated result is shown in Fig. 3.9, and the measured nonthermal electron integrated intensity is shown in parallel for comparison purposes. For a platinum surface
excited by a 10 µJ/mm2 and 55 fs ultrafast pulse, we predict the maximum electron temperature to be 1082 K at 35 fs after zero time delay, i.e., immediately after the laser pulse(55 fs) has totally arrived. The same value for the maximum temperature (about 1000 K)
can be calculated from the measured excitation of the electron gas of 6% [80]. In contrast,
the calculated maximum temperature of the lattice is 174 K, and is reached at 1.2 ps after
the ultrafast pulse excitation. At this long time scale, the electron and lattice temperatures
are equilibrated and the condition of the sample heating is similar to conventional thermal
surface chemical reaction, at which the electron and lattice temperatures are equal.
Figure 3.9 shows that at long delays (> 1 ps), the calculated lattice temperature is alittle higher than the electron temperature. This is a numerical simulation artifact that is

76

1200

Temperature (K)800

Measured Hot Electron
Electron Temperature (Te)
Lattice Temperature (Tp)

600
400
200
0

0

0.5

1

1.5

Measured hot electron intensity (arb unit)

1000

2

Time Delay (ps)Figure 3.9: Calculated electron temperature (Te ) and lattice temperature (Tp ) using
the “two-temperature model” at the platinum (111) surface excited by a
10 µJ/mm2 , 55 fs ultrafast infrared pulse. The measured nonthermal electron integrated intensity (square) is also shown in the figure for comparison
purposes.

77
caused by the simple two-temperature model. This artifact happens because only electron
heat diffusion is considered in Eq. 3.2, and the lattice thermal diffusion is completely
neglected due to the fact that the electron thermal diffusion is significantly more efficientthan the lattice thermal diffusion. The optical excitation of the surface electrons only
happens within the optical penetration depth of the platinum surface, and at around several
picoseconds after the optical excitation the temperature of the surface electrons is stillmuch higher than the bulk. Therefore, the surface electrons keep loosing energy to thebulk electrons to achieve thermal equilibrium for the whole crystal. However, most of
the energy initially adsorbed by the surface electrons has been transferred to the surface
lattice by this time. The surface lattice has to give back its stored energy to the surface
electrons, which continue to loose energy to the bulk electrons. Therefore, the surface
lattice temperature is slightly higher than the surface electron temperature to maintainan energy gradient to give up energy from the surface lattice to the surface electrons forthermal diffusion. True equilibrium will happen at a very long time delay, when the wholesample achieves thermal equilibrium and no gradients remain.
The two-temperature model is often criticized for being too “simple” to describe theactual ultrafast electron heating process. Since the rapid heating of electrons by an intense
ultrafast pulse will cause the surface electrons to become highly excited, the excited electrons cannot be described by a “temperature” that can only be defined when the electrons
are in thermal equilibrium, especially in the first several hundred femtoseconds after theexcitation. In fact, more complicated models are needed to accurately describe the nonthermal electrons excited by ultrafast laser pulses. However, by integrating the nonthermal

78
electrons measured in Fig. 3.5, Fig. 3.9 compares the measured nonthermal electron integrated intensity with the calculated electron temperature. This comparison does show thatthe de-excitation rate of the calculated electron temperature is quantitatively similar to themeasured nonthermal electron intensity. Therefore, the two-temperature model provides
insight into understanding the role of surface hot electrons in femtochemistry on metal
surfaces.
Hot-electron temperature at longer time scales can, in principle, be obtained by measuring the slope of the Fermi-edge from the pump-probe spectrum. However, due to theresolution limitation of our detector and the limited statistics of our measurements, thegradual tilting of the slope of the Fermi-edge cannot be distinguished from our spectrum,
and we are not able to further compare the two-temperature model with the actual measurement at a longer time scale.
3.3.1.3 Excitation of O2 by nonthermal surface electrons
As discussed in Sec. 3.2.4, a transient feature appears at about 6 eV binding energy on
the time scale of about half a picosecond on our measured spectra. This raises a questionof how this transient change is created. There are three possible mechanisms to explain
the transient effect observed, including: (1) pure electronic artifact, (2) phonon-mediatedoxygen state transition, and (3) nonthermal electron-mediated oxygen state transition.
Figure 3.10 compares the integrated intensity of the measured transient feature to thecalculated electron and lattice temperatures by the two-temperature model. Figure 3.10(a)clearly shows that the maximum intensity of the transient feature is delayed about 1 ps withrespect to the time when the maximum electron temperature is reached. Also, the oxygen

79

Oxygen Peak Intensity (arb unit)

(a)

(b)

Oxygen peak intensity
Lattice Temp

0

1

2

3

4

5

Temperature (arb unit)

Oxygen peak intensity
Electron Temp

6Time Delay (ps)Figure 3.10: Integrated intensity of the peroxo signature peak compared to the rescaled (a)electron and (b) lattice temperature.

80
peak relaxes at about 5 ps, by which time the surface electrons have long been cooled forseveral picoseconds. Therefore, if the transient feature is a pure electronic excitation ofcase (1), the peak-like transient feature should have a rise and relaxation time similar tothat of the nonthermal surface electrons. Hence, the possibility that the observed transientfeature is a purely electronic artifact is not likely.
For case (2), it might seem possible that the transient feature could be a phonon mediated process, since the lattice temperature has a rise time similar to that of the transientfeature. However, the lattice temperature actually takes a very long time to cool downwhen compared to the relatively short relaxation time (∼ 5 ps) of the oxygen peak, asshown in Fig. 3.10(b). Therefore, if the observed transient feature was actually driven
primarily by phonon excitation, as in conventional surface chemistry, the relaxation timeof the oxygen peak would be much longer than the 5 ps that we observed. It ought to becomparable to the lattice relaxation time. Furthermore, comparable rise time between thelattice temperature and the transient feature actually indicates that the excitation process
cannot be phonon-mediated. The reason is because a finite time delay is actually required
between the rise times of the lattice temperature and the transient feature, since a finite
coupling time is needed to transfer energy from the lattice to excite the adsorbates, as required by the frictional model [85]. Therefore, the nonthermal electrons at the platinum
surface are most likely to be directly driving the changes in the oxygen configuration.
By excluding the first two possible mechanisms, the transient feature is most likely
mediated by the nonthermal surface electrons of case (3).
3.3.2 Mechanism of O2 excitation

81
3.3.2.1 Identification of the transient feature
From the discussion of Sec. 3.2.2, the difference in the photoemission spectrum at 6 eVbinding energy between superoxo and peroxo chemisorbed states is assigned to the difference in the electron occupancy of the 1πg∗ orbital of the oxygen molecules. For superoxo,
the 1πg∗ orbital is half-filled; whereas for peroxo, the 1πg∗ orbital is fully occupied. This difference changes the density of state of the oxygen molecules and causes the photoemission
spectra of peroxo chemisorbed state to have a peak-like feature at the 6 eV binding energy
below the Fermi-edge [63, 70]. Together with the discussion from the previous section, theappearance of this transient feature possibly reflects a state transition for the chemisorbed
oxygen molecules mediated from the coupling of the nonthermal surface electrons on theplatinum surface.
As mentioned in Sec. 3.1.2, the 1πg∗ orbital of the oxygen molecule is important to theinteraction between the adsorbed oxygen molecules and the platinum surface. Since the1πg∗ orbital is “anti-bonding” in nature, the bond length between the two oxygen atoms
is slightly longer for the peroxo oxygen (fully filled) than the superoxo oxygen (halffilled). However, the 1πg∗ orbital also is hybridized with platinum d-band; thereby it is“bonding” in nature with respect to the oxygen molecule and the platinum surface. This
causes the peroxo oxygen to bond more tightly to the platinum surface. Therefore, as is
widely accepted in conventional investigations, the spectral changes we observed in thepump-probe measurements could not merely reflect electronic transients. It is very likely
to reflect modifications to the chemisorbed state of the oxygen initiated by changes at thesurface-adsorbate bonding.

82
The similarity between the static spectrum of the peroxo oxygen, the static spectrum of
the atomic oxygen (Fig. 3.3), and the transient pump-probe spectrum at about 500 fs timedelays (Fig. 3.6) reveals two possibilities for the nature of this transient state transitionof the adsorbed oxygen molecules. The initial superoxo oxygen (O −2 ) on the platinum
surface can be excited by the ultrafast pulses into either: (1) two transient dissociatedatomic oxygen atoms, or (2) an excited peroxo oxygen (O 2−
2 ).
In fact, these two possible processes are closely related, or even identical to a certain
extent. Previous calculations using density functional theory [69, 86] have suggested thatthe peroxo oxygen can actually be considered to be a “precursor” state in the processof dissociation to two oxygen atoms. According to calculations, superoxo oxygen has
a comparatively higher energy barrier to dissociate; however, after the superoxo oxygen
transfers to the site of peroxo oxygen and subsequently changes its bonding character
through charge transfer with the platinum d-band, the energy barrier for dissociation is
reduced. The reduction of the energy barrier for dissociation can be explained by the
weakening of the intermolecular bond between the two oxygen atoms when an additional
electron is tunneled to the 1πg∗ orbital from the platinum d-band. Moreover, this chargetransfer will cause the oxygen molecule to move from its original bridge site to the threefold-hollow site, due to the creation of an extra bond to the platinum surface.
Complete dissociation into two oxygen atoms is not possible because the transientfeatures are observed in the pump-probe scans are in fact reversible. If the transient feature
is identified as two fully dissociated oxygen atoms, these two oxygen atoms will need torecombine back into an oxygen molecule in a short period of time (∼ 5 ps). In addition,

83
atomic oxygen has the highest binding energy to the platinum surface of all the adsorption
states. Thus it is very unlikely that the stable oxygen atoms would recombine back into
a less-stable molecular configuration. Moreover, the probability for two separate atomic
oxygens to get close enough for recombination would be rather low if the two dissociatedoxygen atoms adsorb at two non-adjacent three-fold-hollow sites (2x2 structure).
Finally, a reversible replenished desorption process is also unlikely. The reason is thatsignificant oxygen re-adsorption to the depleted oxygen sites in the time interval between
the ultrafast pulses (1 ms) requires an oxygen ambient pressure of higher than 10 −3 torr2 ,but the chamber pressure is kept in the 10−10 torr range during the whole experiment. Lateral oxygen diffusion to replenish the depleted oxygen sites is also not possible because thefollowing test was performed to ensure that lateral diffusion rate is low in the experimental
surface temperature (77 K): An oxygen covered surface was first depleted by intense ultrafast pulses. Then, the sample sat still for several minutes to allow possible lateral diffusion
to happen. Intense ultrafast pulses were reintroduced onto the depleted spot, but no further
oxygen desorption could be detected on the second laser desorption. In addition, pictures
taken by STM on an oxygen covered platinum surface cannot identify any possible oxygen
redistribution after the oxygen dosing, indicating oxygen lateral diffusion is insignificant[71].
In conclusion, the transient feature that appears in the pump-probe scans can be identified as the superoxo molecular oxygen state (O−2 ) is transferring into a highly-excited peroxo molecular oxygen state (O2−2 ) or equivalently, a transient dissociated oxygen atomic
2
1 Lamuire (L) ≡ 10−6 torr·sec. In general, dosing 1 L of molecules will deposit a surface to thesaturation coverage. For 1 ms, pressure of 10−3 torr will be required.

84
state (O), in a half-picosecond time scale.
3.3.2.2 Excitation rate of molecular oxygen
By comparing the peak height observed in the pump-probe spectra with that observedin the static spectrum of the atomic oxygen, the actual excitation rate of molecular oxygenby the pump ultrafast pulse can be estimated. As discussed in Sec. 3.2.2, heating an
superoxo oxygen covered platinum sample to different surface temperatures can transformthe oxygen into the peroxo and the atomic oxygen states. For both the TDS and UPS static
spectrum, conventional lattice heating is used to transform the superoxo oxygen to theperoxo and atomic oxygen, and hence the transformed peroxo and atomic oxygen coverageon these two cases should, in principle, be the same.
The total amount of adsorbed oxygen on the platinum surface is proportional to the
combined area of the the α and β peaks from the TDS spectrum. By comparing the ratioof the total areas of the α and β peaks, we estimated that the dissociation ratio d = 25%,
in which 25% of the oxygen molecules of the saturated layer are dissociated thermally.
Then, by comparing the intensity ratio i of the characteristic peak of the peroxo oxygen
pump-probe spectra to the static atomic oxygen spectrum, the total excitation rate inducedby the ultrafast pump pulses is equal to i × d. The ratio of the peak intensities between
the peroxo transient and the atomic static spectra is measured to be i = 70%. Therefore,
the total excitation rate of the peroxo oxygen state is about 70% × 25% = 18%. At
first glance this number seems to be surprisingly high. However, even higher excitation
rates for molecules due to femtosecond laser pumping have been reported. For example,
complete depletion of the adsorbed oxygen molecules by a single intense ultrafast pulse

85
has been reported to the system of O2 /Pt(111) [12], in which the fluence used was several
orders of magnitude higher than in our experiment.
3.3.2.3 Energy transfer mechanism from nonthermal electrons to adsorbates
The excitation mechanism of the adsorbate – the coupling between the excited surface
electron and the adsorbate – can be described on the microscopic scale by means of direct electron (hole) transfer process between the excited surface electron into the LUMO
(HOMO) of the adsorbate.3 The schematic drawing for the orbitals of superoxo and peroxo oxygen is shown in Fig. 3.11 [12, 87]. Several schemes of transfer processes havebeen proposed to explain the desorption processes induced by ultrafast pulses for adsorbates. In particular, for O2 /Pt(111), it has been proposed that the transition is due to theexcited surface electron coupling to the unoccupied 3σu+∗ level [77]; for O2 /Pd(111), holetransferring into the occupied part of the 1πg∗ level is an important process [88].
In the present case, we propose that an excited surface electron is transferred from theplatinum surface to the unoccupied 1πg∗ orbital of oxygen, in which the orbital orientation
is parallel to the platinum surface. There are two justifications:
1. Excitation to the 1πg∗ orbital is the most direct way to achieve the required charge
configuration for the peroxo oxygen, in which its 1πg∗ orbital has one more electron
when compared to the superoxo oxygen [69].
2. For the superoxo oxygen, the πk orbital, which the 1πg∗ hybridized with the platinum
d-band, is only half-filled and is located right at the Fermi-edge. Therefore, theobserved high excitation level for this state transition can be explained because the3

LUMO = Lowest unoccupied molecular orbital, HOMO = Highest occupied molecular orbital

86

3σ+u*

3σ+u*
π⊥∗
Ef

1πg*

π7
π⊥

1πu

1πu

Pt

O2-

O2

Figure 3.11: Because of the increased population of the surface electrons above the Fermi
level induced by the ultrafast pump pulse excitation, the surface electrons
will start to tunnel to the 1πg∗ orbital of the oxygen molecule and this leads toenergy transfer to the oxygen center-of-mass.

87
excited surface electrons has the highest population at the Fermi-edge, and hence
the transition probability for the excited surface electrons tunneling into the the π korbital can be very high. In contrast, since the 3σu∗ orbital lies > 1 eV above theFermi-edge, much fewer surface electrons will be able to reach this energy level forthe same pump pulse intensity. If the electron transition into the 3σ u∗ is the correctmechanism, much higher laser intensities would be expected and be necessary toachieve the electron temperature needed [13].
3.3.2.4 Possible molecular motion during state transition
From the previous discussion, it becomes clear that excitation rate of the adsorbed oxygen by surface electrons reaches its highest level at the maximum temperature of surface
electrons, which is several tens of femtoseconds after the pump pulse. After that, the excitation rate gradually decreases as the surface electrons cool. Therefore, the appearanceof the transient peak-like feature at about 500 fs, which has a significant delay relative tothe maximum electronic excitation of the oxygen molecules, does not reflect the electronic
excitation level. Rather, this transient feature result from changes in the chemisorbed stateaccompanied by motions of the oxygen molecule. In general, the adsorption sites are thelocal minima of the potential energy surfaces (PES): for the superoxo oxygen, it is thebridge site, and for the peroxo oxygen, it is the hollow site. In addition, the PES defines
the route for the transition between two chemisorbed states [69, 70]. For superoxo state
to peroxo state, the oxygen will need to move from the bridge site to the hollow site (seeFig. 3.1). In order to move from the bridge site to the hollow site with the correct molecular orientation and configuration, the oxygen molecular motion could involve rotational

88
and vibrational motions. The oxygen molecule is pulled by the extra bond created when
the hot surface electron transfers to the LUMO state of the oxygen molecule.
Theoretically, the rise time of the transient feature can be related to an effective coupling time τe between the surface electrons and oxygen center-of-mass motion by using
a “frictional model” for the description of substrate-adsorbate interaction [85]. The typical value of the coupling time τe for direct energy exchange between the electron gas andthe adsorbate lies in the range of a few 100 fs to several ps, in good agreement with ourobservations [8, 16, 89].
3.3.2.5 Decay channels for the excited state
The decay of the transient feature in several picoseconds can be interpreted as theactual lifetime of the meta-stable intermediate oxygen state. In case of the peroxo oxygen
transformation, the decay can occur through molecular kinetic energy relaxation scatteringwith the surface electrons and phonons.
For our experiment, since the excited oxygen state (peroxo) is observed to relax to its
ground state (superoxo) in about 5 ps, relaxation to the superoxo oxygen must be the dominant channel; however, it might not be the only possible decay channel. Typically, several
different decay channels are present for conventional thermal excitation; For example, desorption and dissociation often co-exist. For excitation by ultrafast pulses, desorption anddissociation for the adsorbates are always observed together [8, 16, 89]. Therefore, thepermanent feature shown in the photoemission spectrum of Fig. 3.8 after several hoursof ultrafast excitation could likely result from the cumulative effects of slow oxygen desorption and dissociation. Since peroxo oxygen tends to be the preferred configuration for

89(Desorption) O2 + Pt
~ 500 fs
(highly excited)O22-/Pt
O2-/Pt< 5 ps2O /Pt(Dissociation)

Figure 3.12: Possible reaction diagram of how oxygen molecules react after excited by anintense ultrafast pulse on the platinum surface
chemisorbtion at low surface coverage [75], a reasonable explanation for the permanentfeature in valence band may result from slow desorption of oxygen over time and slowly
increase the stable peroxo population on the platinum surface. Another explanation could
be slow dissociation into oxygen atoms by the ultrafast pulses after extended pumping.
However, it is unlikely that surface contamination causes this permanent feature, since thechamber is maintained in ultra high vacuum during the data taking, and the contaminant
is not likely to contribute to the 6 eV peak that we observed.

3.4 Conclusion
After a lengthy discussion, a complete picture of how oxygen molecules adsorbed onPt(111) surface response after the platinum surface is excited by an intense ultrafast pulse
is illustrated in Fig. 3.12 [90, 91]. The oxygen molecules, originally residing on the bridge
site as superoxo state (O−2 ) with one extra electron transferred from the platinum, areexcited by the hot surface electrons to gain another electron from the platinum surface.
After 500 fs, an extra chemical bond between the oxygen and platinum surface is created90
due to the excess electron, and this pulls the oxygen molecule to the hollow site (peroxo−
O2−2 ). In about 5 ps, the excited oxygen will relax back to the bridge site (superoxo O 2 )by giving up the excess electron to the platinum surface. In addition, slow desorption anddissociation happens on a longer time scale and ultimately reduces the molecular oxygen
coverage.

CHAPTER IV

Probing adsorbate oscillation on metal surfaces

To date, studying molecular oscillations on metal surfaces has mainly been done in theenergy domain. In order to measure vibrational modes of molecules absorbed on metal
surfaces, energy loss techniques are typically used. In general, electron energy loss spectroscopy (EELS) is commonly used to measure the vibrational energies of normal modesin the higher energy range; for low energy modes, the technique of helium atom scattering is used instead because the high mass of the helium atom helps to resolve the smallenergy loss resulting from the scattering process. These techniques have successfully measured the vibrational frequencies of adsorbates, i.e. the metal-adsorbate vibrational modeenergies and frustrated rotational mode energies, etc. However, it is not possible using
these techniques to measure instantaneous excitation/de-excitation processes as a resultof external “triggers” to molecules. For example, when a laser pulse irradiates molecules
deposited on a metal surface, vibrational modes of the adsorbates can be triggered either
due to the traditional phonon-induced (cw or nanosecond pulses), or hot surface electroninduced (femtosecond pulses) excitation. Moreover, it is unlikely that scattering experiments can measure how long the adsorbates will take for the vibrational process to start91

92
after the excitation, and the time scale of the corresponding damping process. Therefore,time-domain measurements are needed as a complement to scattering experiment in orderto reveal more information about dynamics.
Molecular vibrations are also very important for surface chemical reactions wheremolecules approach each other to react through various degree of vibrational motions,and electronic rearrangements. Therefore, in order to better understand some of the underlying mechanisms of surface chemistry, time-domain techniques are necessary to follow
reactions at the molecular level in a step-by-step fashion.
Currently, time domain measurements are still very difficult due to a lack of appropriatetechniques with both high surface sensitivity and high time-resolution (fs). In this chapter,
we will demonstrate that by using the TR-UPS technique, and by varying the polarizationof the incoming EUV probe pulses, it is possible to measure the vibrational frequencies ofadsorbates on metal surfaces.

4.1 Photoelectric current affects by molecular symmetry and EUV
polarization
In photoemission spectroscopy (PES), the transition probability of the photoelectric
current produced in the PES process results from the excitation of electrons from an initialstate with wavefunction ψi to a final state with wavefunction ψf by a photon field having~ ( the direction of A
~ is the same as the polarization E~ of the laser
the vector potential A
field). Hence, the transition probability (or photoelectron emission probability) w can beformulated using the Fermi-Golden Rule (or the first Born approximation) which assumes
that the wavelength of the radiation is large compared to the dimensions of the excitation

93
volume [92],w∝

2π|hψf |µ̂|ψi i|2 δ(Ef − Ei − ~ω)
~

(4.1)

where µ̂ = e~r is the permanent electric dipole moment and ~r has the same direction of the~ Certain assumptions about the wavefunctions ψi and ψfpolarization of the laser field E.
can be legitimately applied in our case to simplify the transition matrix element hψ f |µ̂|ψi i.
The two wavefunctions (ψi and ψf ) here are the full wavefunctions describing all theelectrons in the system, and hence Eq. 4.1 is an equation for the many-body problem. For
the first approximation, we can simplify this complicated many-body equation to a simple“one-electron” equation by neglecting effects caused by other electrons. The simplification
for the initial and final state wavefunctions is explained in the following. For the final state,
the adsorbate molecule is in the excited state with one less electron in the atomic core,plus a free electron with kinetic energy Ekin . Therefore, it is reasonable to approximate
the initial-state wavefunction as a product of the single electron orbital φ i,k , from which
the electron k will be excited, and write the wavefunction of the remaining electrons askψi,R(N − 1), for the atom with N total electrons. Therefore, the initial-state wavefunction

can be simplified to be –kψi (N ) = Cφi,k ψi,R
(N − 1)(4.2)

where C is the normalization constant. By following this line of thought, the final state
wavefunction ψf can also be simplified as a product of single-electron wavefunctions
φf,Ekin of the photoemitted free electron and the remaining (N −1) electron wavefunctionsk(N − 1),
ψf,Rk
ψf (N ) = Cφf,Ekin ψf,R
(N − 1)(4.3)

94
Therefore, the transition matrix element of Eq. 4.1 can be separated into two terms,kkhψf |µ̂|ψi i = hφf |µ̂|φi i ψf,R
(N − 1)|ψi,R(N − 1)(4.4)

The first term is the one-electron matrix element, and the second term is the (N − 1) electron overlap integral. The second term generally accounts for many other features in thephotoemission spectrum, such as the core-level shifts and satellite peaks [92, 93], which
are not considered here. Therefore, one can approximate that the remaining orbitals of thekkinitial-state ψi,R
(N − 1), and the final-state ψf,R
(N − 1) to be identical. This approxi-

mation is often called the “frozen-orbital approximation”, causing the second term to bekkapproximated as ψf,R
(N − 1)|ψi,R(N − 1) ≈ 1. The total transition matrix element

now becomes very simple and results in a one-electron matrix element –w ∝ hφf |µ̂|φi i ∝ hφf |~r|φi i(4.5)

Since the photoelectric current measured by detectors is directly proportional to the photoelectron emission probability w, according to Eq. 4.5, the photoemission current is simply
proportional to the integration of the single-electron wavefunctions of the initial orbital φ iand the final free electron wavefunction φf , with the polarization of the applied electricfield ~r.
In this experiment, CO molecules are adsorbed on a platinum(111) surface, where CO
is known to adsorb in the upright position (see next section). The CO molecule is a linear
molecule and belongs to the symmetry point group of C∞v . The corresponding character
table of the point group C∞v is tabulated in Table 4.1. According to Eq. 4.5 and by using~ the photoelectron emission
the terminology of group theory, for a given polarization E,

95
C∞v
A1 ≡ Σ +A2 ≡ Σ −E1 ≡ Π
E2 ≡ ∆
E3 ≡ Φ
E4 ≡ Γ
..
.

E
1
1
2
2
2
2

Γx,y,z

3

φ
2C∞
1
1
2 cos(φ)
2 cos(2φ)
2 cos(3φ)
2 cos(4φ)

2φ
2C∞
1
1
2 cos(2φ)
2 cos(4φ)
2 cos(6φ)
2 cos(8φ)

3φ
2C∞
1
1
2 cos(3φ)
2 cos(6φ)
2 cos(9φ)
2 cos(12φ)

···
···
···
···
···
···
···
..
.

∞σv
1
−1
0
0
0
0

1 + 2 cos(φ) 1 + 2 cos(2φ) 1 + 2 cos(3φ) · · ·1zRz(x, y), (Rx , Ry )Table 4.1: Character table of the symmetry group C∞v . (z is the direction of the principle
symmetric axis)
probability w 6= 0 only when the direct product of the irreducible representations of thethree functions (φi , φf , and ~r) contains the totally symmetric irreducible representation 1(Σ+ in the case of C∞v ) [94]. Mathematically,Γφi ⊗ Γ~r ⊗ Γφf 3 Σ+

(4.6)

Γφi ⊗ Γ~r = Γφf(4.7)

or equivalently [95],where Γφi denotes the irreducible representation of the function i. For normal emission of
the photoelectrons, the emission direction is along the principle symmetric axis z, whichcorresponds to the irreducible representation Σ+ , according to Table 4.1. Since irreducible
representation Σ+ is totally symmetric, this simplifies Eq. 4.6 that w 6= 0 if the direct
products of the Γφi ⊗ Γ~r contains Σ+ .
The polarization of the EUV probe pulses then affects the symmetry of Γ~r . According
to Table 4.1, for p-polarized EUV pulses, where the polarization of the pulses is parallel to the plane of incidence (z in the character table), Γ~r = Σ+ . For s-polarized EUV
1

An irreducible representation is a totally symmetric irreducible representation when the characters forall symmetric operations is equal to 1.

96
C∞v
Σ+
Σ−
Π
∆
Φ
Γ
..
.

Σ+
Σ+

Σ−
Σ−
Σ+

Π
Π
Π
Σ+ + [Σ− ] + ∆

∆
∆
∆
Π+Φ
Σ+ + [Σ− ] + Γ

Φ
Φ
Φ
∆+Γ
Π+H
Σ+ + [Σ− ] + I

Γ
Γ
Γ
Φ+H
∆+I
Π+Θ
Σ+ + [Σ− ] + K

Table 4.2: Table of the direct product (⊗) of the symmetry group C ∞v
pulses, the polarization of the pulses is perpendicular to the plane of the incidence ( x ory in the character table) and Γ~r = Π. Table 4.2 is the direct product table for the symmetry group C∞v . According to this table, for an orbital with σ symmetry (Γφi = Σ+ ),p-polarized EUV pulses ( Γφ~r = Σ+ ) are needed to have a non-zero photoelectron emission probability (Σ+ ⊗ Σ+ = Σ+ ). On the other hand, for an orbital with π symmetry(Γφi = Π), s-polarized EUV pulses (Γφ~r = Π) are required (Π ⊗ Π = Σ+ + [Σ− ] + ∆ contains Σ+ ). Therefore, by changing the polarization of the EUV pulses, molecular orbitals
with different symmetries can be selectively enhanced or suppressed in the photoemission
spectrum. More detail discussion about the relation between the angular pattern of emitted
photoelectrons and the laser polarization can be found in Ref. [95].

4.2 Symmetry of the orbitals of CO/Pt(111)
As mentioned in the previous paragraphs, CO is adsorbed on the Pt(111) surface forthis experiment. The platinum surface is routinely cleaned with argon sputtering and oxygen annealing. After the platinum surface is cleaned, 5 L of CO is introduced onto thesurface through a gas doser to achieve a saturation CO coverage of 0.5 ML. At this coverage, the CO molecules equally occupy the bridge νb (C − O) = 232 meV and atop sites

···

97

2π∗

5σ

1π

C
2π∗

4σ

O

3σ
1π

(a)
metal

chemisorbed CO
2π∗free CO2π∗5σ∗p-stateEfd-band
2π
5σ

5σ

(b)

1π

1π

4σ

4σ

Figure 4.1: Schematic diagram showing the bonding character of a CO molecule adsorbed
on a metal surface. (a) The CO-metal bond is provided by the most extended
orbital of the molecule which is the 5σ orbital. Charge back donation fromthe metal into the formerly empty 2π ∗ orbitals is indicated by thin arrows. (b)Schematic energy level diagram of the CO-metal system. The energy levels on
both sides are for the isolated CO molecule and the metal surface. The middle
diagram illustrates the energy levels of an adsorbed CO molecule. Band splitting for the adsorbed CO happens for the 5σ and 2π ∗ levels due to hybridizaton
with the metal d-band and the p-orbitals. (figures reproduced from Ref. [92])98
νt (C − O) = 258 meV, producing a c(4 × 2) LEED pattern [75, 13]. The CO molecules
adsorb in an upright position with the intermolecular axis perpendicular to the metal surface, with the carbon atom at the bottom bonded to the metal surface, while the oxygen
atom resides on top. Figure 4.1(a) illustrates the outermost molecular orbitals of the COmolecule and indicates how the CO molecule binds to the platinum surface. The energetically highest occupied orbital and thus also the spatially most extended one is the 5σ
orbital which is located mainly on the carbon atom. Therefore, it is intuitively reasonable
that the CO molecule tries to attach its “softest” part to the metal surface in the surface
chemical bond. The initial step in binding is therefore a 5σ-to-metal charge transfer. Thischarge transfer is compensated partly by back donation of charge from the metal surface
onto the CO molecule. This back donated charge is into an orbital that is a mixture of the2π ∗ of the free CO molecule with the metal d-band [96]. It is interesting to note that in
the free CO molecule, the 2π ∗ orbital is about 1.5 eV above the vacuum level Ev ,and the
hybridizaton with the metal d-band causes it to locate below Ev [97].
Figure 4.1(b) illustrates the energy levels of the orbitals for free CO molecules andchemisorbed CO molecules on metal surfaces. When CO molecules are brought into contact with a metal surface, the 5σ orbital of the CO molecule interacts with the p-orbitalof the metal atoms, splitting the 5σ orbital into a bonding and an anti-bonding state. The
resulting bonding state is energetically only slightly higher than 1π orbital, in contrast to
its much higher energy position originally. The 2π ∗ orbital also interacts with the d-band
causing a splitting into a bonding and an anti-bonding state. The bonding state is locatedwell below the Fermi-edge Ef in energy, and hence provides a back donation channel for99

Photoelectron Signal (arb. unit)

2.5

1π/5σ

2.0

4σ

1.5

1.0

Polarization
S
P

0.5

0.0
-5

0

5

10

15

20

25

30

35

Binding Energy (eV)Figure 4.2: Photoemission spectrum of CO molecules adsorbed on a platinum(111) single
crystal surface. Spectra are taken with high harmonic photons (42 eV) withdifferent s (black) and p (red) polarizations.
charge transferring from platinum to the CO molecule.
As pointed out in the previous section, the orbital symmetry of the CO molecules
results in different photoemission cross-sections depending on the polarization of the exciting EUV light. Figure 4.2 plots the photoemission spectra of CO on Pt(111) using s andp polarized EUV probe pulses without pump beam excitation, i.e. the so-called “static”
spectra. The two spectra show significant similarities in both the high and low binding
energy regions. However, in the energy range from 5 to 15 eV, noticeable differences
are caused by the different polarizations of the EUV probe pulses. Comparing with theenergy level diagram of the chemisorbed CO on Fig. 4.1(b), the peak at 9 eV binding energy can be assigned to the 1π/5σ orbital combination – the energy lowering of the 5σ

100
orbital caused by hybridizaton with the metal d-band, results in a 5σ bonding state that is
indistinguishable with the 1π orbital in energy. As shown in Fig. 4.2, there are changes inintensity for the 1π/5σ peak when using different EUV polarizations. However, since the1π and the 5σ orbitals are so close in energy after adsorption, it is difficult to use this peak
to analyze the CO orientation.
There is another peak at a 12 eV binding energy that can be assigned to the 4σ orbital.
It has very different emission cross-sections for the two different EUV polarizations. As
shown in Fig. 4.2, the 4σ peak is greatly suppressed for s-polarized light, as expected dueto the symmetry selection rule explained in the previous section. This kind of complementary behavior of σ and π emissions was first observed by Plummer et al. for CO/Ni(111)[98] and has been used to determine the geometry of the adsorbed molecules on metal
surfaces [95]. There is still a little reminant of the 4σ peak in the case of s-polarized light.
This is because the TOF detector is mounted about 30o off the surface normal. Although
the ideal case would be to mount the TOF detector normal to the sample, the off-normal
mounting has a negligible affect on the photoemission yield for the 4σ peak and therefore
does not affect the overall experiment.

4.3 Resolving oscillations of adsorbed CO molecules
In the previous sections, it has been shown that there is a dependence of the photoelectron emission cross-section on the EUV polarization. Therefore, it is possible to use thisproperty to measure the oscillation period of the adsorbate after excitation by an ultrafast
pulse. After the surface is excited by an ultrafast pulse, hot electron tunneling from the substrate to the adsorbate can occur. The excited electrons raise the potential energy surface

101

θ
Low Yield

EEUV
O O

High Yield

CC

Pt
(a)

Photoelectron signal (arb. unit)

1.5

1.4
4σ

1.3

1.2

1.1
0fs
400fs
1.0

0.9

(b)

10

12

14

16

18

Binding energy (eV)Figure 4.3: (a) Schematic diagram illustrating that when the CO molecules are alignedwith the polarization of the EUV pulse, the photoemission cross-section is
maximized. After excitation by an ultrafast laser pulse, the CO molecules start
to vibrate. When the CO molecules slightly non-parallel to the EUV polarization, the photoemission cross-section is reduced. (b) Pump-probe spectra close
to the 4σ peak. These spectra are taken when the polarization of the EUV probe
pulses is perpendicular to the platinum surface (p-polarized) , and the polarization of the excitation pump pulses are parallel to the surface (s-polarized) with
15µJ/mm2 /pulse fluence. Spectra of 0 fs and 400 fs time delays are shown forcomparison, showing that the the 4σ peak in both height and area at 0 fs timedelay are bigger than those at 400 fs time delay.

102
of the adsorbate and transfer energy to the atomic center-of-mass – causing the molecule
to vibrate, or even desorb, depending on the excitation energy. For CO on Pt(111), the 2π ∗orbital is close to the Fermi-edge Ef . Hot electrons can tunnel to 2π ∗ orbital to transfer energy, in a process similar to DIET/DIMET. Actually, inverse photoemission spectroscopy(IPES) has measured the 2π ∗ orbital to be about 1.5 eV above the Fermi-edge for CO on
Ni(111) [99], providing a channel for electronic tunneling to the CO. By fixing the polarization of the laser pulse in one particular polarization, e.g. p-polarized, as illustrated
in Fig. 4.3(a), the CO molecules could start to vibrate. According to Eq. 4.5, the photoemission cross-section of a particular orbital is directly related to the overlap between thepolarization of the EUV pulses and the “direction” of the wavefunction φ i,k of that orbital;
or in the other words, the photoemission cross-section w of the CO 4σ bond is a functionof how well the bond lines up with the laser polarization. By using this simple idea, theoscillation period of the CO molecules can readily be measured by performing a series ofpump-probe scans by following the intensity changes of the 4σ peak.
Figure 4.3(b) shows the pump-probe spectra at time delays of 0 fs and 400 fs aroundthe energy region of the 4σ peak. It is clear that the 4σ peak at 400 fs time delay is lower
in both height and area when comparing to the 4σ peak at 0 fs time delay. This change
is expected because at 0 fs time delay, the CO molecular axis should still be parallel tothe polarization of the EUV light, giving a maximum intensity for the 4σ peak. At a timedelay of 400 fs, the CO molecules are no longer parallel to the polarization of the EUV
light, and the photoemission signal decreases accordingly.
The oscillation period of the CO molecules can be obtained by integrating the area of103

Integrated Peak Area (arb. unit)-0.05

950 fs

-0.10

-0.15

-0.20

-0.25

950 fs
0

500

1000

1500

2000

Time Delay (fs)Figure 4.4: Integrated area of the 4σ peak of the CO/Pt(111) plotted at various time delays,
showing that the CO molecules are oscillating at the platinum surface after
pump pulse excitation.

104
the 4σ peak as a function of time delay between pump and probe. The spectra at various time delays are first subtracted from a reference spectra at -100 fs, and the remaining
signals are then integrated over the binding energy of the 4σ peak. By plotting these integrated areas versus time delay, a nice oscillatory curve is obtained, as shown in Fig. 4.4.
The oscillation period is measured to be about 950 fs, when measured either at the maxima or minima of the oscillation signal. Also, in Fig. 4.4 the oscillation signal tends todecrease at longer delay times. This effect is expected since the vibrational energy of the
CO molecule can slowly transfer to the platinum surface, reducing its oscillation amplitude
accordingly.
Further information can be revealed at the Fermi edge. Fig. 4.5 shows the photoemission spectra at the Fermi-edge at the time delays of 0, 100, 200, and 500 fs respectively.
It is obvious that the photoemission yield at 0 fs is significantly higher than for the restof the photoemission spectra. This indicates that a significant population of non-thermal
electrons are created by the pump pulse. These non-thermal electrons are still distinguishable at 100 fs, but by 200 fs, the electron distribution has cool significantly. This is similar
to our previous observation on oxygen at Pt(111) – the hot electrons distribution is significantly reduced by about 200 fs.

4.4 Discussion
4.4.1 CO normal modes on a platinum surface
In the gas phase, a molecule can have 3N degrees of freedom, which can be divided
into three translational, 3 rotational and 3N − 6 vibrational modes (for linear molecules,
2 rotational and 3N − 5 vibrational modes). N is the number of atoms in the molecules.

105

0.14

Photoelectron signal (arb. unit)

0.12

0.10

0.08

0.06

0.04

0fs
100fs
200fs
500fs

0.02

0.00
-6

-5

-4

-3

-2

-1

0

Binding Energy (eV)Figure 4.5: Pump-probe spectra near the Fermi-edge, showing that hot electrons are
rapidly excited when the ultrafast pump pulses excites the surface. They subsequently relax in about 200 fs. There is a static component resulting from asmall residual 29th harmonic.

106

O
A

1

ν

1

C

O
A1 ν1

M

C

O

O
A

1

ν

2

C

A1 ν2

O

3

C
M

M

E ν

B

1

ν

O

B

1

ν

O
4

C

C

M
O

M

3

C

E ν4

M

M

M

B

2

ν

M

M
O

5

B

2

ν

M
O

6

C

C

C

M

M

M

On-top Site

Bridge Site

Figure 4.6: Schematic representation of the normal modes of on-top and bridge-bonded
CO on platinum surface. The relative length of the arrows represent the movements of the carbon and oxygen atoms. (figure adapted from Ref. [100] andRef. [101])

107
When molecules adsorb on surfaces, all translational and rotational motions are restricted,
so the 3N degrees of freedom are all vibrational modes. These vibrational modes are bestclassified as 3 frustrated translations, in which the molecule, as a whole, moves perpendicular or parallel to the surface, 3 (2 for a linear molecule) frustrated rotations, and finally
3N − 6 (3N − 5for linear molecule) vibrational modes.
For classification purposes, point group symmetry can be used to associate the CO adsorption to the top site and the bridge site on the platinum surface. The top site gives theC4v point group, and the bridge site gives C2v point group. Fig. 4.6 shows schematicallythe form of localized vibrational modes for CO on Pt(111) on the on-top and bridging position. In each case, there are two modes belonging to the totally symmetric A 1 representation,
which have their dipole moment changes normal to the surface. Hence, ν 1 is closely related to the free molecule C-O stretching mode. ν2 is a hindered translation mode normal
to the platinum surface, which becomes essentially a Pt-C stretching mode. The remaining four modes – the x and y frustrated translations and the frustrated rotations appear as
degenerate pairs for the top site CO molecules; but in the bridge site, the degeneracy islifted and results in two modes, ν3 and ν4 , in the plane of the bridge site, and two modes,
ν5 and ν6 , perpendicular to the plane of the bridge site.
These vibrational modes can be measured using energy loss methods. In general, infrared spectroscopy (IRS) and electron energy loss spectroscopy (EELS) have been employed successfully for measuring adsorbate vibrational frequencies with energies above
200 cm−1 [102, 103]. For vibrational energy lower than this range, a helium atom scattering technique is employed to investigate the low frequency modes [104, 105, 101]. In

108

C-O stretch ν1
Pt-CO stretch ν2
CO frustrated rotation ν3
CO frustrated translation ν4

normal node energy (cm−1 ) oscillation period (fs)on-top site
bridge site
on-top site bridge site
2100
1871
16
18
480
361
72
92
411 *
81
48
59
696
566

Table 4.3: Normal mode energies of CO adsorbed on Pt(111) (* calculated for CO/Ni)
particular, for CO adsorbed on Pt(111) at a low coverage of CO (≤ 1/3 mono-layer),
CO molecules mainly occupied on the on-top sites. The high frequency C-O stretchingmode ν1 is measured to be 2100 cm−1 (258 meV), and the Pt-C stretching mode ν2 is
at a relatively lower frequency, estimated to be 480 cm−1 (58 meV) by EELS [75, 106].
When the CO coverage increases to 0.5 mono-layer and above, the bridge sites will start
to be filled by the CO molecules. Therefore, the EELS spectrum will start to develop
two new additional loss peaks at slightly reduced energies at 1871 cm −1 (232 meV) and
361 cm−1 (45 meV), corresponding to the C-O stretching mode and the Pt-C stretchingmode at the bridge sites respectively. The CO frustrated rotation mode ν 3 has been calculated for CO adsorbed on Ni, and the calculated value is 411 cm −1 (51 meV), which isin the same order of magnitude as the ν2 mode. (For the other two modes calculated for
CO/Ni: ν1 = 2088 cm−1 (259 meV), and ν2 = 437 cm−1 (54 meV). These are similar to
the measured value of CO adsorbed on Pt, and hence, it is reasonable to believe that the ν 3of CO/Pt(111) has a similar value) [100]. Finally, for the frustrated translation mode ν 4 ,helium scattering experiment measured the value to be 48 cm −1 (6 meV) for the top site,
and 59 cm−1 (7 meV) to the bridge site [101, 107].
The oscillation periods of these modes can be calculated from the measured energies.
Assuming that all normal modes of the adsorbates are harmonic oscillators, then the vi-

109brational energies can be written as
1
En = (n + )~ω
2

(4.8)

where n is the quantum number of the normal mode, and ω = 2π/τ is the angular frequency, and τ is the oscillation period. For energy loss experiments, the measured energy
is the energy difference between the ground state (v = 0) and the excitation state (v = 1),
and hence, the vibrational period can be converted from the measured energy byτ = h/Emeasured(4.9)

The energies of the four normal modes of the the CO/Pt(111) and their corresponding
converted oscillation periods are tabulated in Table 4.3.
Considering all of the vibrational modes of CO/Pt(111), the oscillation period corresponding to the frustrated translation mode ν4 (τ4 = 696 and 566 fs) is the closest one tothe period we measured using TR-UPS (τTR−UPS ' 950 fs). All the other mode periods
are more than an order of magnitude shorter in time than the period measured. In fact, thefrustrated translation mode ν4 and the frustrated rotation mode ν3 should be most sensitive to the polarization of the EUV pulses, due to the fact that these modes directly affectthe orientation of CO molecules on the platinum surface, and subsequently affect the orientation of the 4σ orbital. For the frustrated rotation mode ν3 , one would expect anotheroscillation period on the order of 100 fs should be observed and that this fast period should
be superimposed on the slow varying oscillation period of ν4 . However, preliminary studyof measuring the variation of the 4σ peak with a finer time resolution does not reveal another oscillation feature, indicating the possibility that the ν3 mode might not be as easily

110
6σ∗

ν4

2π∗

Ef

ν1

O

2π

ν2
5σ

2π∗

e-

1π

C

2π∗

e-

4σ

(a)

Pt(111)CO(b)

Figure 4.7: (a) When an ultrafast pulse excites the platinum surface, hot electrons are created on the surface. The hot electrons then tunnel to the 2π ∗ orbital of the COmolecule. (b) Due to the bonding character to the platinum surface of the 2π ∗orbital, the C-O vibrational mode ν1 and the CO-Pt vibrational mode ν2 will
be induced. The frustrated translation mode ν4 could either be directly excitedby the hot electron, or subsequently excited due to energy coupling between
modes.
excited as the frustrated translation ν4 . In fact, in energy loss spectroscopy, in our knowledge, the ν3 mode has not been observed experimentally. This might be an indication thatthe potential energy surface of the CO/Pt(111) system might prohibit the excitation of theCO frustrated rotation mode ν3 effectively.
4.4.2 Hot electrons mediated excitation
The excitation mechanism of the ν4 mode related to hot surface electrons that are created and sustained within the first 200 fs after pump pulse excitation. As shown in Fig. 4.4,
the oscillation begins immediately after pump pulse excitation, and the 4σ orbital reduces
its intensity significantly by a 200 fs time delay. According to the two temperature model

111
discussed in the last chapter, the surface electrons respond to ultrafast pulse excitation
almost instantaneously through directly adsorbing the ultrafast pulse energy which could
subsequently raises the electron temperature to several thousand Kelvin, depending on thepump pulse intensity and pulse-width. However, the surface lattice cannot directly respondto the ultrafast pulse excitation, and remains relatively “cold” for picoseconds. If the CO
excitation is phonon-mediated, as in conventional surface chemistry, a time delay of picoseconds would be expected before the onset of oscillation. Therefore, once again, thehot surface electrons created by ultrafast pulses play an important role in the CO excitation
observed.
For CO on Pt(111), the unoccupied 2π ∗ orbital is close to the Fermi-level. Since thephoton energy of our pump pulse is 1.5 eV, the hot surface electrons will be excited to
1.5 eV above the Fermi-edge. Therefore, these hot electrons will have sufficient energy toreach this unoccupied CO orbital through tunneling, as shown in Fig. 4.7. In fact, the 2π ∗orbital is an anti-bonding orbital with respect to the bonding between carbon and oxygen
atom; on the other hand, occupying the 2π ∗ orbital would enhance the bonding strength
between the platinum surface and the CO molecule.
Therefore, after the hot surface electron transfers to the 2π ∗ orbital of the CO molecules,
the potential energy surface (PES) of the CO molecules is suddenly excited through FanckCondon excitation. The nuclei of the CO molecules are not positioned at the energy minimum of the excited PES, and the nuclei will be driven along the excited PES to new
positions. Therefore, the distance between the carbon atom and the oxygen atom will be
increased, and the distance between the CO molecule and the platinum surface will be-112come shorter. Typically, these hot electrons will only stay on the adsorbates for a timescale of about 10 fs, as discussed in the studies of unoccupied state lifetime of adsorbates
using two-photon photoemission techniques from Sec. 1.3. After the hot electrons leavethe 2π ∗ orbital of the CO, the PES will be relaxed to the ground-state, and the carbon andoxygen atoms are now required to reverse their courses. Because of these motions, theenergy of the hot electrons is partly transferred to kinetic energy of the nuclei. Hence electron transition processes result in launching vibrations for two normal modes ν 1 and ν2 .
This picture is very similar to the DIET/DIMET process of desorption by ultrafast pulse.
Hot surface electrons transfer energy to molecular motion through the excited potential
energy surface by Franck-Condon excitation, as discussed in the appendix.
The excitation of the frustrated translation mode ν4 could be due to subsequent mode
coupling between the highly excited modes ν1 and ν2 with ν4 , or could be due to direct
excitation by the hot surface electrons, or even a mix of the two mechanisms. If the former
is true, then the coupling time between the normal modes should be fairly “fast” in order
to explain the quick onset of the observed oscillation signal. Further investigation on thisprocess will be needed in order to fully understand the excitation mechanism of the ν 4
mode.
Finally, the oscillation period we measured using TR-UPS is about 950 fs, which
is longer than the numbers measured by helium atom scattering experiments (696 and
566 fs). In fact, for the technqiue of time-resolved vibrational sum-frequency generation,
as discussed in Sec. 1.5.2, a strong transient redshift of the vibrational energy for the C-O
stretch mode has been observed after ultrafast pulse excitation, . The vibrational energy

113
is shifted from ∼ 2020 cm−1 to ∼ 1980 cm−1 and persists for more than 20 ps, as shownin Fig. 1.10(b). They explained this effect is cause by anharmonic coupling of the C-O
strech mode to low-frequency frustrated modes [36, 35, 108, 109]. Further investigation isneeded to clarify how this energy redshift happens.

4.5 Conclusion
In this chapter, measuring the vibrational period of an adsorbate in the time domain has
been demonstrated using the symmetry of the molecular orbitals and the polarization of
the EUV ultrafast pulses. In particular, the CO frustrated translation mode ν 4 is measured
to be about 950 fs, similar to results obtained using helium scattering techniques in theenergy domain. Actually, CO translational motion has been proposed to be very importantto the CO oxidation process on metal surfaces, as well as the rotational motions of theO2 molecules, as reported in the last chapter. Density functional calculations [68, 110,103] have suggested that in the beginning of the CO oxidation process, oxygen molecules
rotate from the bridge site to the hollow site in order to elongate the intermolecular bond
between the two oxygen atoms. This results in dissociation into two oxygen atoms withlower dissociation energy. Simultaneously, CO molecules translate and bend towards theoxygen molecules (or atoms) to “attract” one of the oxygen atoms to recombine with it
and finally leave the surface as CO2 . If this picture of CO oxidation is correct, then thetwo measurements that we performed of O2 /Pt(111) and CO/Pt(111) support observationof the first steps in the suspected mechanism of CO oxidation.

CHAPTER V

Future work

In previous chapters, we have demonstrated that TR-UPS is a powerful tool in theinvestigation of chemical dynamics of adsorbates on metal surfaces. This technique has
great potential for revealing many of the intermediate steps of surface chemical reactions
and their corresponding time scales. Thanks to this capability, future studies based on
TR-UPS may provide significant insights into the mechanisms behind many catalytic processes. The results from these studies may give theorists a better theoretical understandingof catalytic processes on the atomic level. However, to date, both experimental and theoretical studies of the dynamics of surface chemical reactions are still in their infancy,
and tremendous efforts will be required in this emerging field in the future. Hopefully,
someday, the understanding will be mature enough to apply the knowledge obtained to engineering processes leading to cheaper and more robust catalysis. In the following chapter,several proposed improvements to the experimental setup and possible future work will be
discussed. Our hope is that these ideas will be realized in our laboratories in the coming
two to three years.

114

115

5.1 Time-resolved core-level spectroscopy
The photon energy of the high-harmonic generated EUV pulses used in our current
TR-UPS setup is 42 eV (30 nm wavelength). The reasons for choosing this photon energy
are its relatively high flux, and ease of generation, with relatively less driving laser intensity required. The 42 eV photon energy is sufficient for probing valence-band structure;
however, liberating electrons from core-level orbitals will require higher photon energy.
For example, the binding energies of the 1s (deep core-level) and 2s (shallow core-level)
orbitals for oxygen atoms are 543 and 41.6 eV, respectively [111].
Core-level photoemission spectroscopy (or XPS) is a common technique for determining both physisorbed and chemisorbed states of adsorbates on metal surfaces. Since photoelectrons from the core levels are very sensitive to the surrounding chemical environment,
XPS is widely used to study static physical phenomena on surfaces using synchrotrons.
UPS is generally used to observe changes in molecular valence bands, which gives information about the dynamics of adsorbate bonding structure. XPS, on the other hand, isused to observe energy shifts of the core-level electrons, giving site-specific information
about molecules in different chemical environments. Therefore, pushing the EUV photon
energy to higher values will allow us to obtain more information about the system, using both UPS and XPS. Information obtained using each technique can help us to better
understand the dynamics of chemical reactions.
Generally speaking, photon energy in the range of 30 to 200 eV has good surface sensitivity, since the escape depth of electrons in this energy range is under 5 Å [2]. The
current record high-harmonic-generation photon energy is 460 eV (2.7 nm) using helium,

116
and 239 eV (5.2 nm) using neon, using a pulsed gas jet and tera-watt ultrafast pulses [3].
However, though high-harmonic generation in these high energy ranges is possible, efficient high-harmonic generation has only been demonstrated for photon energies on theorder of 50 to 100 eV. To efficiently generate photons of higher energies, higher laser intensity is needed. The high laser intensity, however, leads to a higher degree of ionizationof the interacting gas molecules, preventing the driving laser and the EUV light from propagating at the same speed, severely limiting the conversion efficiency. Recent results fromour group have shown that by using a modulated hollow-core waveguide to periodicallyvary the intensity of the driving laser pulses, one can achieve efficient EUV generationeven in the presence of substantial ionization. The immediate result of this work is tothe shifting of the phase-matching condition to even higher photon energies. About 20 pJ
generated intensity has been obtained for individual harmonics at around 100 eV photon
energy using this corrugated capillary method [5]. In addition, high quality multi-layer
mirrors are much easier to fabricate around 95 eV (13 nm) photon energy with mirror reflectivity better than 75% achievable, in contrast to only 25% reflectivity at around 42 eV(30 nm). Therefore, although high harmonic generation at 100 eV will have ∼ 1 order ofmagnitude less flux than at 42 eV, higher reflectivity multi-layer mirrors can compensate
for the lower generated flux, giving a comparable detected signal level.

5.2 Probing bimolecular chemical reaction on surfaces
The work presented in chapter III demonstrated that TR-UPS allows direct observation of chemical reactions on metal surfaces. We saw that when oxygen molecules rotatefrom the bridge site to the three-fold-hollow site on platinum, a covalent bond between the

117
surface and the oxygen is created, weakening the intra-molecular bond between the twooxygen atoms. This type of bond creation is generally considered to be one of the simplest
chemical reactions. However, completely time-resolving bimolecular chemical reactions
on surfaces has long been a major challenge for surface chemists. In particular, by identifying all the precursor states and their reacting time-scales, the reaction “bottleneck” canthen be pin-pointed, perhaps improving the efficiency of the chemical reactions.
CO oxidation (2CO + O2 → 2CO2 ) on platinum surfaces is a good candidate system for such experiments. Theoretical calculations have been performed on CO oxidationon the platinum(111) surface [103, 68]. These calculations suggest that peroxo oxygen
molecules (O2−2 ) have a lower activation barrier for reacting with CO molecules than superoxo oxygen molecules (O−2 ). This is possibly due to the fact that peroxo oxygen has alonger O-O intra-molecular bond, and since peroxo oxygen molecules have a less corrugated potential energy surface around the hollow site, the more preferable site for chemical
reactions [68]. Therefore, the catalytic channel for the platinum surface is likely to be thatthe platinum surface provides a dissociation pathway for the oxygen molecules, allowing oxygen molecules to move from the bridge site to the hollow site, and lowering theirdissociation energy. Subsequently, the intermolecular bond cleaves, creating two excitedoxygen atoms. The excited oxygen atoms can then directly react with CO molecules, finishing the oxidation process. Finally, the CO2 molecule will leave the platinum surface,
releasing the adsorption sites for another O2 and CO molecule pair to undergo the catalytic
process again.
If this picture of CO oxidation is correct, observing this process could be possible using

118
TR-UPS. One possible scheme for using TR-UPS to obverse the spectral changes could be
to look for the characteristic peak of the peroxo oxygen to appear about half a picosecond
after the pump pulse arrives, indicating the movement of the oxygen molecules from thebridge site to hollow site. In the meantime, the 4σ and 1π/5σ features of CO molecules
could be used to monitor the orientation of CO molecules. In addition, the strength of the
CO peaks should diminish for longer times, due to desorption of the CO 2 molecules fromthe platinum surface after the oxidation process. Therefore, hopefully, CO oxidation may
be time-resolved using TR-UPS (or TR-XPS).

5.3 Angle- and time-resolved photoemission spectroscopy
Currently, the TR-UPS setup cannot resolve the emission angles of photoelectrons
emitted from the sample surface. In many surface science experiments, resolving the emission angle is not critical to understanding the details behind the mechanism under study.
However, for some experiments, knowing angular information will significantly improve
our understanding of the experimental details. For example, in the experiment observing
CO oscillation periods on a platinum surface (chapter IV), photoelectrons originating fromthe 4σ orbital would have an emission peak along the intra-molecular axis. Therefore, if
photoelectron angular information were available, a clearer picture of the CO molecularoscillation would emerge, allowing, for example, precise determination of the oscillation
angle of the CO molecules.
In synchrotron-based photoemission experiments, two-dimensional hemispherical electron analyzers have recently been used to obtain precise angular distributions of emitted
photoelectrons. Traditionally, hemispherical electron analyzers have not been useful in ob-

119taining photoelectron angular distributions. In addition, these analyzers can only measure
the photoelectron kinetic energy over a narrow energy range, and are unable to simultaneously measure a range of kinetic energies. This results in the loss of most of the photoelectrons. Therefore, these kinds of energy analyzers are only suitable for very bright lightsources, such as synchrotron radiation sources. State-of-the-art two-dimensional hemispherical electron analyzers use a sophisticated electronic lens to preserve angular information, while using an MCP and a CCD camera at the detection end for two-dimensional
data retrieval. The kinetic energy and emission angle of a photoelectron are mapped onto asingle spot of the two-dimensional image, allowing the detector to simultaneously record
a certain angular range and kinetic energy range of the photoelectrons. Thanks to such
advancements, these hemispherical analyzers could be used to replace the current timeof-flight detector, providing photoelectron angular distribution information without losing
the photoelectron count rate in a certain energy range. Currently, these new hemispherical electron analyzers can measure up to about ±7◦ photoemission angle (±15◦ is underdevelopment), while simultaneously recording a 10 eV energy range with about 100 meVenergy resolution [112].
Another possible design implementing angle-resolved photoemission spectroscopy borrows from the technique of “cold-target recoil-ion momentum spectroscopy” (COLTRIMS)[113, 115, 116]. COLTRIMS is used in ion-scattering experiments to simultaneously measure the kinetic energies and emission angles of ejected electrons and recoil-ions after ion
collision. The key idea of COLTRIMS is the use of a strong magnetic and/or electric field
inside the experimental apparatus to confine the flight paths of the ejected electrons and

120

(a)

(b)

Figure 5.1: (a) Schematic drawing of “cold-target recoil-ion momentum spectroscopy”(COLTRIMS). A two-stage supersonic gas jet is installed to provide a welldefined atomic beam, which collides with an energetic ion-beam. Helmholtz
coils and resistive coated parallel plates are used to generate homogeneous
magnetic and electric fields to confine the recoil-ions and the ejected electrons.
Position-sensitive micro-channel plates (MCP) are used to resolve the kinetic
energy and emission angles of the recoil-ions and electrons. (figure reproduced
from Ref. [113]) (b) The anode of the position-sensitive MCP is wrapped bya fine copper-wire (delay-line) on each dimension. When an electron strikes
the anode, the position of the electrons on the detector can be extracted bymeasuring the time difference of the electronic signals at both ends of eachdelay-line. The position for each dimension can be determined independently.
(figure reproduced from Ref. [114])

121
recoil-ions. The confined electrons and ions undergo rotational motion due to the magnetic field confinement, and their kinetic energies and emission angles can be evaluatedby measuring their flight times along with the their impact positions on the detector. A
time- and position-sensitive detector is used to capture both the flight time and detection
position simultaneously. Such kind of detectors can be realized by using a MCP scalerwith its anode wrapped by a two-dimensional delay-line array. These detectors are commercially available [114]. By measuring the time difference of the electron impact signals
at both ends of each delay-line, the position of the electron impact can be determined. It
is possible to adapt this idea for angle-resolved photoelectron spectroscopy by putting a
metal surface into the interaction region, then measuring the kinetic energies and emission
angles of the photoelectrons.

APPENDICES

122

123

APPENDIX A

Theoretical formalism of adsorbate desorption induced byultrafast pulses on metal surfaces

The theory of ultrafast laser-induced chemical reactions on metal surfaces is still under development, nevertheless it is beneficial to review current theory in explaining thisemerging “femtochemistry” field. In particular, the desorption of adsorbates from metal
surfaces has been studied for about a decade, both experimentally and theoretically. Theories based on inelastic scattering from nonthermal surface electrons have been used to
explain this unconventional desorption process. Furthermore, as was shown experimentally in chapter III and IV, non-thermal surface electrons also play an important role inexcitation of chemisorbed states, and molecular vibrational modes. Therefore, a brief review of current theories of desorption can give insight to help understand various possible
explanations.

A.1

Desorption induced by electronic transition (DIET)

Conventional thermal-driven chemical reactions on metal surfaces are thought to bea process of the adsorbate molecules overcoming the reaction energy barrier required fordesorption, by progressively acquiring energy from the surface lattice phonons. However,

124
femtochemistry on metal surfaces cannot be explained using traditional theories. New theories have been proposed to explain the underlying mechanism of this new phenomenon.
When an energetic electron inelastically scatters with a molecule in the gas phase,the probability of vibrationally exciting the molecule through electron scattering is wellknown to be greatly increased if the incident electron can be trapped into a “shape resonance”. This shape resonance corresponds to a particular orbital of the gas-phase moleculethat forms a temporary negative molecular ion [117, 118, 119, 120, 121]. This can be explained by “Franck-Condon” principle [122]: When an electron is trapped into a molecularorbital, the “Born-Oppenheimer” (BO) or “Adiabatic” approximation [123] can be applied.
Under this approximation, atomic motion is considered to be “frozen” while the electronictransition is occurring; i.e., the motion of electrons is much faster than the motion of theatomic nuclei. The sudden change of the molecule’s Hamiltonian due to this excited statetransition instantaneously effects the forces on the nuclei, causing the molecule to vibrate.
This theory can be used to describe the process of adsorbate desorption from metal surfaces via intense ultrafast pulses. An intense laser field generates hot surface electrons on ametal surface, and these electrons can tunnel into the unoccupied states of the adsorbates,
initiating the process of desorption.
The effectiveness of resonant excitation depends on two important factors. First, theincident electrons must be trapped in a “bond-significant” orbital; that is, one whose occupancy provides new forces on the atomic constituents. In this way, the equilibriumgeometry of the negative ion differs from that of the original neutral target. It is very
important that the electron be trapped in order to efficiently transfer energy via Franck-

125
Condon excitation. Under these conditions, the excited potential energy surface (PES) of
the atomic space, modified by the trapped electrons, sets the atoms in motion. In contrast,
pure inelastic scattering between energetic electrons and atoms does not transfer enoughenergy from the electron to the atom because the atom is much heavier than the electron
in weight.
Second, the effectiveness of this resonant bond excitation depends strongly on the lifetime of the resonant negative-ion state relative to a characteristic time scale for “significant” nuclear motion on the excited PES. For example, for adsorbate desorption from metal
surfaces, the highest possibility of molecular desorption occurs when the adsorbate-metalbond is at its point of greatest compression or expansion. In this situation, the lifetime of
the trapped ion will be close to one-half of the vibrational period of the adsorbates.
A.1.1 Resonant excitation by non-thermal surface electrons
On metal surfaces, nonthermal surface electrons created by ultrafast pulses become theequivalent of energetic incoming electrons to the molecules in the gas phase. Similar toconventional thermal surface chemistry, the temperatures of the surface electrons and thephonons (lattice) reach an equilibrium at all times. In femtochemistry, energy from theultrafast pulses heats the surface electrons within the first picosecond, while the motion of
the surface lattice is slow. Thus, the lattice remains unperturbed some picoseconds after theexcitation. Figure A.1 is a schematic diagram showing the energy level of an adsorbate thatis adsorbed onto a metal surface. The highest occupied molecular orbital (HOMO) of theadsorbate is partially filled by electrons, up to the Fermi-level, while the lowest unoccupied
molecular orbital (LUMO) of the adsorbate is left unfilled. In general, there is an energy

126
barrier between the adsorbate and the metal surface, which is created by the space-groupsymmetry broken on the metal surface. This barrier prevents the metal’s surface electrons
freely propagating into the adsorbate orbitals [123]. Before laser excitation, the adsorbate
is in equilibrium with the metal surface and resides in the ground state PES of the system.
When an ultrafast pulse hits the metal surface, individual surface electrons will adsorb
one photon from the ultrafast pulse, raising their kinetic energy by hν. Since metals haveno band gap, electrons can exist at a range of energies above the Fermi-edge (from ε F ermi
to εF ermi + hν), as shown in Fig. A.1. These excited electrons produce a photoelectron
current ≡ jel (k(εin ; hν)) from within the surface which is incident upon the adsorbed
molecules. If the energy level of the LUMO state of the adsorbates falls within this energy
range, the nonthermal surface electrons, having acquired sufficient energy, can transfer(or tunnel) to the adsorbate molecules by the process of resonance scattering. Typically,
the incident electrons will reside in this resonance state for a time τR , and then scatterback into an unoccupied conduction band state with energy ε f ≤ εi , leaving the adsorbate
excited with a net energy ∆ε = εi − εf .
When an electron scatters into the LUMO state, the adsorbates’ Hamiltonian changes
suddenly. Thus, the adsorbates are excited from the ground state PES to an excited state
PES. The BO approximation tells us that the electronic state of the adsorbate system determines the PES for the adsorbate nuclei. Therefore, when an extra electron couples
to the LUMO state of the adsorbate, the adsorbate is suddenly excited to an excitedstate PES, and atomic forces subsequently cause the nuclei of the adsorbates to move.
The results is atomic motion, mediated by non-thermal electrons, can lead to a cleav-

127

hν

εin
εfin

LUMO

hν

εFermiHOMO

Metal Substrate

Adsorbate

Figure A.1: Energy level diagram showing that the non-thermal electrons are energized
rapidly by an intense ultrafast pulse. These “hot” electrons could scatter into
the LUMO state of the adsorbates to transfer energy to excite the molecularmotions through Franck-Condon excitation scheme. (figure reproduced from
Ref. [79])128
age of the bond between the adsorbates and the surface, ultimately leading to desorption.
This desorption scheme is generally called “Desorption Induced by Electronic Transition”(DIET).[124, 125]A schematic diagram of the DIET process, depicted in Fig. A.2, shows the PES (onedimensional curve for simplicity) associated with the electronic ground and excited states.
There are two scenarios under which DIET can occur, depending on the nature of theadsorbates’ excited-state PES.
Fig. A.2(a) depicts the most straight forward DIET process, in which the adsorbates
enter a repulsive excited-state PES. After the adsorbate enters the excited-state PES, characterized by an repulsive potential Vexcite , the adsorbate is accelerated on an outward trajectory. Under this scenario, three outcomes are possible. The first case (and most typical)
is a rapid relaxation of the adsorbate, falling back onto the ground-state PES V 0 , with aslightly displaced from the equilibrium position z0 . The adsorbate has picked some kinetic
energy by temporally residing on Vexcite , but not enough to overcome the energy barrierfor desorption. Classically, this means that relaxation occurs prior to the adsorbates’ trajectory arriving at the diabetic curve crossing point z = zc . The second possible outcome,and the major desorption channel for the adsorbate, occurs when the adsorbate resides
on Vexcite long enough to cross the point zc , later relaxing back to V0 with kinetic energyε∗ = Vexcite (z0 ) > εinf
0 , i.e., enough to overcome the energy barrier for desorption. The
final case, which is rare, occurs when the adsorbate stays on the excited-state PES until
complete desorption happens.
Another scenario of the DIET process, which is strongly dependent on the adsorbate

129

V0

Vexcite(a)

ε0inf
εexciteinf
z0

V0

zc

Vexcite

(b)

ε0inf

z0

Figure A.2: Potential curves illustrating the process of Desorption Induced by Electronic
Transition (DIET). (a) Intra-molecular electronic excitation involving boundto-repulsive excitation; (b) Intra-molecular electronic excitation involving attractive intermediate state mediation. (figure reproduced from Ref. [79])130
motion over Vexcite , is illustrated in Fig. A.2(b). Here the excited-state PES is more closely
bonded to the surface than the ground-state PES. When this happens, the adsorbate will
be pulled towards the metal surface following the electronic transfer. Consequently, theinitial accelerated nuclear motion is directed inwards such that when the excited state
decays back to the ground state (usually within a fraction of a vibrational period), not only
has the adsorbate acquired kinetic energy but, perhaps more importantly, it is returned highup on the repulsive wall of the ground-state PES, with a compressed bond between surface
and adsorbate. Depending upon the parameters, while the adsorbate is in the intermediate
excited-state PES, it may or may not obtain sufficient kinetic energy to desorb once backin the ground-state PES. Both situations (successful desorption and remaining adsorbed)
are shown in Fig. A.2(b).
A.1.2 Desorption probability
The energy-resolved production rate for adsorbates with total energy ε, due to resonance scattering by the non-thermal surface electrons is written as d Ṅ (ε; τR , hν)/dε. The
non-thermal surface electrons are scattered from state ki to state kf , with energy εi andεf = εi − ε ≥ εF ermi , respectively [79]. This give usXdṄ(ε; τR , hν) =jel (ki ; hν)σT ot (εi (ki ); εf (kf ); τR )δ(εi − εf − ε)
dε
k ,k
i

(A.1)

f

In this expression, τR is the propagation time the adsorbate spends on the excited-state PES
V− (z) before decaying back to the ground-state PES Va (z). jel (ki ; hν) is the flux distribution of non-thermal surface electrons incident upon the adsorbates. The flux is characterized by the wave-vector ki and depends on the laser photon energy hν. σT ot (εi (ki ); εf (kf ); τR )is the total inelastic resonance scattering cross-section, which depends on the incident en-

131ergy εi , and exit energy εf of the electron, and the propagation time τR . Equation A.1
can be simplified under some reasonable assumptions, (concerning relatively weak opticalexcitation [79]), todṄ
(ε; τR , hν) ≈ QRes (ε, hν)P (ε, τR )
dε

(A.2)

where P (ε, τR ) is the total probability distribution for exciting the molecule with energy
ε. A graphical description of P (ε, τR ) is shown in Fig. A.3. QRes is the electronic factor,
which is assumed to be slowly varying over the energy spread of P (ε, τ R ). Here, theelectronic factor QRes is approximated to be a constant, and includes all the averaged
effects of the incident flux jel and the scattering cross-section σT ot that occur during thescattering process. The distribution P (ε, τR ) can be evaluated using various microscopic
theories. The “Gaussian wavepacket model” is commonly used as it provides a physicallyintuitive picture of the desorption mechanism using features of classical mechanics while
retaining many quantum attributes.
Using the “Gaussian wavepacket model”, the distribution P (ε, τ R ) can be written as
[126]
"

2 #
1
1 ε − εf (τR )√ exp −P (ε; τR ) =2∆(τR )∆(τR ) 2π(A.3)∆(τR ) ≡ ūVa0 (zτR )[1 + (ωτR )2 ]1/2

(A.4)

p2τR
+ Va (zτR )εf (τR ) =
2M

(A.5)

with

and

Equation A.4 shows that the width of the energy distribution after the wavepacket returns

132

V-(z)

KE

t=τr

ε∗
t=0

ε
D

Va(z)
KE

D
P(ε)

Figure A.3: Potential energy curves of center-of-mass translational motion of thechemisorbed ion (charge transfer from surface) with respect to the surface,showing wavepacket propagation throughout the time sequence involving thenegative-ion LUMO resonance. The distribution of the final adsorbate states,
between vibrationally excited bound (white) and desorptive continuum states(black) is shown as P (ε) versus ε. (figure reproduced from Ref. [79])133
to the ground-state PES Va (z) at τR is determined by the combination of ū = (~/2M ω)1/2 ,the spread of the original (t = 0) oscillator probability distribution Va0 , the slope of Va atthe point of return, and the additional width acquired while propagating on V − .
The desorption probability for the adsorbates can then be determined by integrating
P (ε; τR ) from the desorption barrier D to infinity. This is
Pdes (τR ) =Z ~νP (ε; τR )dε

(A.6)

D

which for ~ν  D reduces toD − εf (τR )Pdes (τR ) ' 0.5 1 − erf √2∆(τR )

A.2

(A.7)

Desorption induced by multiple electronic transition (DIMET)

The desorption rate Pdes (τR ) of the adsorbates depends on the propagation time τR ,as the adsorbates undergo motion on the excited-state PES in this time scale to obtainkinetic energy to overcome the desorption barrier D. In general, the propagation time τ R
can be interpreted as the time that non-thermal electrons spend in the LUMO state of theadsorbate, and is in general very short. Therefore, most adsorbates will not be able toobtain enough kinetic energy to escape from the well upon returning to the ground-state
PES Va (z). This does not explain the non-traditional exponential increase in desorption
with ultrafast laser excitation.
In the conventional DIET scheme, most of the adsorbates will not contribute to thedesorption yield as not enough kinetic energy is generally obtained in a single excitation
process. In DIMET [11, 127], on the other hand, adsorbates could be re-excited before its
kinetic energy gained through the previous DIET process is totally relaxed, due to the high134

Figure A.4: Trajectories for motion on the ground and excited PES. The dotted lines represent the total energy of the adsorbate complex, with vertical segments arising from Franck-Condon transitions and horizontal segments from conservative motion on a given PES. (a)(b) Trajectories displayed associated with asingle-excitation DIET process. (c)(d) Trajectories for the multiple-excitation
DIMET process. (figure reproduced from Ref. [11])concentration of non-thermal surface electrons. This situation is shown in Fig. A.4(c) and(d). The adsorbate makes another Franck-Condon transition, this time from a vibrationallyexcited level of the ground-state PES. The additional energy associated with the vibrational
excitation allows the adsorbate to desorb more easily previously discussed excitations.
However, if the excitations are separated by more than the lifetime of the adsorbate-surface
vibration, the DIMET mechanism is simply several single DIET excitations, which will
not lead to enhancement of the desorption yield.

BIBLIOGRAPHY

135

136

BIBLIOGRAPHY[1] S. Backus, C. Durfee, M. Murnane, and H. Kapteyn. High power ultrafast lasers.
Rev. Sci. Instrum., 69(3):1207–1223, 1998.
[2] G. A. Somorjai. Intorduction to surface chemistry and catalysis. John Wiley &Sons, 1994.
[3] Z. Chang, A. Rundquist, H. Wang, M. M. Murnane, and H. C. Kapteyn. Generationof coherent soft x-rays at 2.7 nm using high harmonics. Phys. Rev. Lett., 79(1620):2967–2970, 1997.
[4] C. Durfee, A. Rundquist, S. Backus, C. Herne, M. Murnane, and H. Kapteyn.
Phase matching of high-order harmonics in hollow waveguides. Phys. Rev. Lett.,
83(11):2187–2190, 1999.
[5] A. Paul, R. A. Bartels, R. Tobey, H. Green, S. Weiman, I. P. Christov, M. M. Murnane, H. C. Kapteyn, and S. Backus. Quasi-phase-matched generation of coherence
extreme-ultraviolet light. Nature, 421(6918):51–54, 2003.
[6] F. J. Kao, D. G. Busch, D. Cohen, D. Gomes da Costa, and W. Ho. Femtosecond
laser desorption of molecular adsorbed oxygen from Pt(111). Phys. Rev. Lett.,
71(13):2094–2097, 1993.
[7] J. A. Prybyla, T. F. Heinz, J. A. Misewich, M. M. T. Loy, and J. H. Glownia. Desorption induced by femtosecond laser pulses. Phys. Rev. Lett., 64(13):1537–1540,
1990.
[8] L. M. Struck, L. J. Richter, S. A. Buntin, R. R. Cavanagh, and J. C. Stephenson.
Femtosecond laser-induced desorption of CO from Cu(100): Comparison of theory
and experiment. Phys. Rev. Lett., 77(22):4576–4579, 1993.
[9] G. Eichorn, M. Richter, K. Al-Shamery, and H. Zacharias. Time-correlated laser
desorption of NO from NiO(100)/Ni(100). Chem. Phys. Lett., 289(3-4):367–372,
1998.
[10] F. Budde, T. F. Heinz, M. M. T. Loy, J. A. Misewich, F. de Rougemont, andH. Zacharias. Femtosecond time-resolved measurement of desorption. Phys. Rev.
Lett., 66(23):3024–3027, 1991.

137[11] J. A. Misewich, A. Kalamarides, T. F. Heinz, U Höfer, and M. M. T. Loy.
Vibrationally assisted electronic desorption: Femtosecond surface chemistry of
O2 /Pd(111). J. Chem. Phys., 100(1):736–739, 1994.
[12] S. Deliwala, R. Finlay, J. Goldman, T. Her, W. Mieher, and E. Mazur. Surface
femtochemistry of O2 and CO on Pt(111). Chem. Phys. Lett., 242(6):617–622,
1995.
[13] W. D. Mieher and W. Ho. Bimolecular surface photochemistry: Mechanisms of cooxidation on Pt(111) at 85 K. J. Chem. Phys., 99(11):9279–9295, 1993.
[14] F. J. Kao, D. G. Busch, D. Gomes da Costa, and W. Ho. Femtosecond versus
nanosecond surface photochemistry – O2 +CO on Pt(111) at 80K. Phys. Rev. Lett.,
70(26):4098–4101, 1993.
[15] T. Her, R. J. Finlay, C. Wu, and E. Mazur.
Surface femtochemistry of
CO/O2 /Pt(111): The importance of nonthermalized substrate electrons. J. Chem.
Phys., 108(20):8595–8598, 1998.
[16] M. Bonn, S. Funk, C. Hess, D. Denzler, C. Stampfl, M. Scheffler, M. Wolf, and
G. Ertl. Phonon- versus electron-mediated desorption and oxidation of CO on
Ru(0001). Science, 285(5430):1042–1045, 1999.
[17] W. Steinmann and Th. Fauster. Two-photon photoelectron spectroscopy of electronic states at metal surfaces. In H. L. Dai and W. Ho, editors, Laser spectroscopy
and photochemistry on metal surfaces, Advanced series in physical chemistry, chapter 5. World Scientific Publishing, 1995.
[18] S. Schuppler, N. Fischer, Th. Fauster, and W. Steinmann. Lifetime of imagepotential states on metal surfaces. Phys. Rev. B, 46(20):13539–13547, 1992.
[19] R. W. Schoenlein, J. G. Fujimoto, G. L. Eesley, and T. W. Capehart. Femtosecond
studies of image-potential dynamics in metals. Phys. Rev. Lett., 61(22):2596–2599,
1988.
[20] R. W. Schoenlein, J. G. Fujimoto, G. L. Eesley, and T. W. Capehart. Femtosecond relaxation dynamics of image-potential states. Phys. Rev. B, 43(6):4688–4698,
1991.
[21] R. W. Schoenlein, J. G. Fujimoto, G. L. Eesley, and T. W. Capehart. Femtosecond
dynamics of the n=2 image-potential state on Ag(100). Phys. Rev. B, 41(8):5436–
5439, 1990.
[22] M. Bauer, S. Pawlik, and M. Aeschlimann. Decay dynamics of photoexcited alkali
chemisorbates: Real-time investigations in the femtosecond regime. Phys. Rev. B,
60(7):5016–5028, 1999.
[23] M.Bauer, S. Pawlik, and M. Aeschlimann. Resonance lifetime and energy of an
excited Cs state on Cu(111). Phys. Rev. B, 55(15):10040–10043, 1997.

138[24] M. Bauer, S. Pawlik, R. Burgermeister, and M. Aeschlimann. Symmetry propertiesof an electron alkali excitation at a noble metal surface as investigated by twophoton photoemission. Surf. Sci., 402-404:62–65, 1998.
[25] M. Bauer, S. Pawlik, and M. Aeschlimann. Femtosecond lifetime investigations of
excited adsorbate states: atomic oxygen on Cu(111). Surf. Sci., 377-379:350–354,
1997.
[26] Th. Fauster and W. Steinmann. Photonic probes of Surface, page 347. NorthHolland, Amsterdam, 1995.
[27] H. Petek, M. Weida, H. Nagano, and S. Ogawa. Real-time observation of adsorbate
atom motion above a metal surface. Science, 288(5470):1402–1404, 2000.
[28] S. Ogawa, H. Nagano, and H. Petek. Phase and energy relaxation in an antibonding
surface state: Cs/Cu(111). Phys. Rev. Lett., 82(9):1931, 1999.
[29] A. G. Borisov, A. K. Kazansky, and J. P. Gauyacq. Stabilisation of alkai-adsorbateinduced states on Cu(111) surfaces. Surf. Sci., 430(1-3):165–175, 1999.
[30] W. S. Fann, R. Storz, H. W. K. Tom, and J. Bokor. Electron thermalizaton in gold.
Phys. Rev. B, 46(20):13592–13595, 1992.
[31] P. B. Allen. Theory of thermal relaxation of electrons in metals. Phys. Rev. Lett.,
59(13):1460–1463, 1987.
[32] W. S. Fann, R. Storz, H. W. K. Tom, and J. Bokor. Direct measurement of nonequilibrium electron-energy distributions in subpicsecond laser-heated gold films. Phys.
Rev. Lett., 68(18):2834–2837, 1992.
[33] J. Bokor and W. S. Fann. Ultrafast hot electron relaxation in metals. In H. L. Dai
and W. Ho, editors, Laser spectroscopy and photochemistry on metal surfaces, Advanced series in physical chemistry, chapter 8. World Scientific Publishing, 1995.
[34] J. A. Prybyla, H. W. K. Tom, and G. D. Aumiller. Femtosecond time-resolved
surface reaction: Desorption of CO from Cu(111) in < 325 fs. Phys. Rev. Lett.,
68(4):503–506, 1992.
[35] M. Bonn, C. Hess, S. Funk, J. Miners, B. Persson, M. Wolf, and G. Ertl. Femtosecond surface vibrational spectroscopy of CO adsorbed on Ru(001) during desorption. Phys. Rev. Lett., 84(20):4653–4656, 2000.
[36] Ch. Hess, M. Wolf, S. Roke, and M. Bonn. Femtosecond time-resolved vibrationalsfg spectroscopy of CO/Ru(001). Surf. Sci., 502-503:304–312, 2002.
[37] Y. R. Shen. Surface nonlinear optics: a historical prospective. IEEE J. Sel. Top.
Quant., 6(6):1375, 2000.
[38] Y. R. Shen. Surface-properities probed by 2nd-harmonic and sum-frequency generation. Nature, 337(6027):519–525, 1989.

139[39] B. N. J. Persson, F. M. Hoffmann, and R. Ryberg. Influence of exciton motion on
the shape of optical absorption lines: Applications to vibrations at surfaces. Phys.
Rev. B, 34:2266–2283, 1989.
[40] R. W. Schoenlein, W. P. Leemans, A. H. Chin, P. Volfbeyn, T. E. Glover, P. Balling,
M.S Zolotorev, K.-J. Kim, S. Chattopadhyay, and C. V. Shank. Femtosecond xray pulses at 0.4 Å generated by 90o thomson scattering: A tool for probing thestructural dynamics of materials. Science, 274(11):236–238, 1996.
[41] W. P. Leemans, R. W. Schoenlein, P. Volfbeyn, A. H. Chin, T. E. Glover, P. Balling,
M.S Zolotorev, K.-J. Kim, S. Chattopadhyay, and C. V. Shank. Interaction of relativistic electrons with ultrashort laser pulses: generation of femtosecond x-rays andmicroprobing of electronbeams. IEEE J. Quant. Electr., 33(11):1925–1934, 1997.
[42] K.-J. Kim, S. Chattopadhyay, and C. V. Shank. Generation of femtosecond x-rays
by 90o thomson scattering. Nucl. Instrum. Meth. A, 341(1-3):351–354, 1994.
[43] C. Pellegrini, J. Rosenzweig, G. Travish, K. Bane, R. Boyce, G. Loew, P. Morton, H.-D. Nuhn, J. Paterson, P. Pianetta, T. Raubenheimer, J. Seeman, R. Tatchyn,
V. Vylet, H. Winick, K. Halbach, K.-J. Kim, M. Xie, D. Prosnitz, E. T. Scharlemann, R. Bonifacio, L. De Salvo, and P. Pierini. The SLAC soft x-ray high power
FEL. Nucl. Instru. Meth. A, 341(1-3):326–330, 1994.
[44] G. N. Kulipanov, A. N. Skrinsky, and N. A. Vinokurov. Sychrotron light sources
and recent developments of accelerator technology. J. Synchr. Rad., 5(3):176–178,
1998.
[45] K.-J. Kim. Advanced capabilities for future light sources.
5(3):202–207, 1998.

J. Synchr. Rad.,[46] D. Attwood, editor. Soft X-rays and extreme ultraviolet radiation. Cambridge University Press, 1999.
[47] P. A. Franken, A. E. Hill, C. W. Peters, and G. Weinreich. Generation of opticalharmonics. Phys. Rev. Lett., 7(4):118–119, 1961.
[48] A. L’Huillier, M. Lewenstein, and P. Balcou. Harmonic generation in strong laser
fields. Cambridge University Press, 2004.
[49] J. Diels and W. Rudolph, editors. Ultrashort Laser Pulse Phenomena. Academic
Press, 1996.
[50] A. L’Huillier and Ph. Balcou. High-order harmonic generation in rare gases with a
1-ps 1053-nm laser. Phys. Rev. Lett., 70(6):774–777, 1993.
[51] P. B. Corkum. Plasma perspective on strong-field multiphoton ionization. Phys.
Rev. Lett., 71(13):1994–1997, 1993.

140[52] J. L. Krause, K. J. Schafer, and K. C. Kulander. High harmoinc generation fromatoms and ions in the high intensity regime. Phys. Rev. Lett., 68(24):3535–3538,
1992.
[53] K. J. Schafer and K. C. Kulander. High harmoinc generation from ultrafast pumplasers. Phys. Rev. Lett., 78(4):638–641, 1997.
[54] R. Trebino, editor. Frequency-Resolved Optical Gating: The measurement of Ultrashort Laser Pulses. Kluwer Academic Publishers, 2002.
[55] L. Nugent-Glandorf, M. Scheer, D. A. Samuels, V. Bierbaum, and S. R. Leone. A
laser-based instrument for the study of ultrafast chemical dynamics by soft x-rayprobe photoelectron spectroscopy. Rev. Sci. Instrum., 73(4):1875–1886, 2002.
[56] L. Nugent-Glandorf, M. Scheer, D. A. Samuels, V. Bierbaum, and S. R. Leone. Ultrafast photodissociation of Br2 : Laser-generated high-harmonic soft x-ray probingof the transient photoelectron spectra and ionization cross sections. J. Chem. Phys.,
117(13):6108–6116, 2002.
[57] Website of the center of x-ray optics, Lawrence Berkely National Laboratory.
http://cindy.lbl.gov.
[58] A. Stolow. Gigahertz bandwidth ultrahigh vacuum 50 Ω coaxial high-voltage coupling capacitor for photoelectron spectroscopy. Rev. Sci. Instrum., 67(5):1777–
1780, 1996.
[59] J. A. R. Samson. Techniques of vacuum ultraviolet spectroscopy. John Wiley &Sons, 1967.
[60] R. B. Cairns and J. A. R. Samson. Metal photocathodes as secondary standards
for absolute intensity measurements in the vacuum ultraviolet. J. Opt. Soc. Am.,
56(11):1568–1573, 1966.
[61] J. L. Wiza. Microchannel plate detectors. Nucl. Instrum. Meth., 162(1-3):587–601,
1979.
[62] M. Drescher, M. Hentschel, R. Kienberger, G. Tempea, C. Spielmann, G. A. Reider,P. B. Corkum, and F. Krausz. X-ray pulses approaching the attosecond frontier.
Science, 291(5510):1923–1927, 2001.
[63] C. Puglia, A. Nilsson, B. Hernnas, O. Karis, P. Bennich, and N. Martensson. Physisorbed, chemisorbed and dissociated O2 on Pt(111): Studied by different corelevel spectroscopy methods. Surf. Sci., 342(1-3):119–133, 1995.
[64] W. Wurth, J. Stöhr, P. Feulner, X. Pan, K. R. Bauchspiess, Y. Baba, E. Hudel,
G. Rocker, and D. Menzel. Bonding, structure, and magnetism of physisorbed andchemisorbed O2 on Pt(111). Phys. Rev. Lett., 65(19):2426–2429, 1990.

141[65] J. Gland, B. Sexton, and G. Fisher. Oxygen interactions with the Pt(111) surface.
Surf. Sci., 95(2-3):587–602, 1980.
[66] W. Eberhardt, T. Upton, S. Cramm, and L. Incoccia. Inner valence levels as astructural probe for chemisorbed molecules: O2 on Pt(111). Chem. Phys. Lett.,
146(6):561–565, 1988.
[67] S. Lehwald, H. Ibach, and H. Steininger. Overtones and multiphoton processes invibration spectra of adsorbed molecules. Surf. Sci., 117(1-3):342–351, 1982.
[68] A. Eichler and J. Hafner. Reaction channels for the catalytic oxidation of CO on
Pt(111). Surf. Sci., 433-435:58–62, 1999.
[69] A. Eichler and J. Hafner. Molecular precursors in the dissociative adsorption of O 2
on Pt(111). Phys. Rev. Lett., 79(22):4481–4484, 1997.
[70] A. Eichler, F. Mittendorfer, and J. Hafner. Precursor-mediated adsorption of oxygenon the (111) surfaces of platinum-group metals. Phys. Rev. B, 62(7):4744–4755,
2000.
[71] B. Stipe, M. Rezaei, and W. Ho. Atomistic studies of O2 dissociation on Pt(111)induced by photons, electrons, and by heating. J. Chem. Phys., 107(16):6443–6447,
1997.
[72] B. Stipe, M. Rezaei, W. Ho, S. Gao, M. Persson, and B. Lundqvist. Single-molecule
dissociation by tunneling electrons. Phys. Rev. Lett., 78(23):4410–4413, 1997.
[73] N. R. Avery. An EELS and TDS study of molecular oxygen desorption and decomposition on Pt(111). Chem. Phys. Lett., 96(3):371–373, 1983.
[74] T. Matsushima. The mechanism of the CO2 formation on Pt(111) and polycyrstalline surfaces at low temperatures. Surf. Sci., 127(3):403–423, 1983.
[75] H. Steininger, S. Lehwald, and H. Ibach. Adsorption of oxygen on Pt(111). Surf.
Sci., 123(1):1–17, 1982.
[76] M. L. Bocquet, J. Cerdà, and P. Sautet. Transformation of molecular oxygen
on a platinum surface: A theoretical calculation of STM images. Phys. Rev. B,
59(23):15437–15445, 1999.
[77] R. J. Finlay, T. H. Her, C. Wu, and E. Mazur. Reaction pathways in surface femtochemistry: routes to desorption and reaction in CO/O2 /Pt(111). Chem. Phys. Lett.,
274(5-6):499–504, 1997.
[78] J. W. Gadzuk. Hot-electron femtochemistry at surfaces: on the role of multiple
electron processes in desorption. Chem. Phys., 251:87–97, 2000.
[79] J. W. Gadzuk. Laser Spectroscopy and Photochemistry on Metal Surfaces, chapter 21, pages 897–942. World Scientific, Singapore, 1995.

142[80] B. Rethfeld. private communication.
[81] P. Siffalovic, M. Drescher, M. Spieweck, T. Wiesenthal, Y. C. Lim, R. Weidner,
A. Elizarov, and U. Heinzmann. Laser based apparatus for extended ultraviolet
femtosecond time-resolved photoemission spectroscopy. Rev. Sci. Instrum., 72:30–
35, 2001.
[82] E. Zarate, P. Apell, and P. M. Echenique. Calculation of low-energy-electron lifetimes. Phys. Rev. B, 60:2326–2332, 1999.
[83] R. Knorren, K. H. Bennemann, R. Burgermeister, and M. Aeschlimann. Dynamics of excited electrons in copper and ferromagnetic transition metals: Theory andexperiment. Phys. Rev. B, 61:9427–9440, 2000.
[84] S. I. Anisimov, B. L. Kapeliovich, and T. L. Perelman. Electron-emission fromsurface of metals induced by ultrashort laser pulses. Sov. Phys. JEPTP, 66:776,
1974.
[85] F. Budde, T. F. Heinz amd A. Kalamarides, M. M. T. Loy, and J. A. Misewich. Vibrational distributions in desorption induced by femtosecond laser pulses - couplingof adsorbate vibration to substrate electronic excitation. Surf. Sci., 283:143–157,
1993.
[86] P. D. Nolan, B. R. Lutz, P. L. Tanaka, J. E. Davis, and C. B. Mullins. Molecularly
chemisorbed intermediates to oxygen adsorption on Pt(111): A molecular beamand electron energy-loss spectroscopy study. J. Chem. Phys., 111(8):3696–3704,
1999.
[87] I. Panas and P. Siegbahn. A theoretical study of the peroxo and superoxo forms ofmolecular oxygen on metal surfaces. Chem. Phys. Lett., 153:458–464, 1988.
[88] J. Misewich, S. Nakabayashi, P. Weigand, M. Wolf, and T. Heinz. Anomalous
branching ratio in the femtosecond surface chemistry of O2 /Pd(111). Surf. Sci.,
363(1-3):204–213, 1996.
[89] T. Germer, J. Stephenson, E. Heilweil, and R. Cavanagh. Hot-carrier excitationof adlayers – time-resolved measurement of adsorbate-lattice coupling. Phys. Rev.
Lett., 71(20):3327–3330, 1993.
[90] M. Bauer, C. Lei, K. Read, R. Tobey, J. Gland, M. M. Murnane, and H. C. Kapteyn.
Direct observation of surface chemistry using ultrafast soft-x-ray pulses. Phys. Rev.
Lett., 87(2):25501–25504, 2001.
[91] C. Lei, M. Bauer, K. Read, R. Tobey, Y. Liu, T. Popmintchev, M. M. Murnane,and H. C. Kapteyn. Hot-electron-driven charge transfer processes on O 2 /Pt surface probed by ultrafast extreme-ultraviolet pulses. Phys. Rev. B, 66(24):245420–
245430, 2002.
[92] S. Hüfner. Photoelectron spectroscopy. Springer-Verlag, 1995.

143[93] H. J. Freund and N. Neumann. Photoemission of molecular adsorbates. Appl. Phys.
A, 47:3–23, 1988.
[94] D. C. Harris and M. D. Bertolucci, editors. Symmetry and spectroscopy: an introduction to vibrational and electronic spectroscopy. Dover publications, New York,1978.
[95] H. P. Steinrück. Angle-resolved photoemission studies of adsorbed hydrocarbons.
J. Phys.: Condens. Matter, 8:6465–6509, 1996.
[96] G. Blyholder. CNDO model and interpretation of the photoelectron spectrum of
CO chemisorbed on Ni. J. Vac. Sci. Technol., 11(5):865–868, 1974.
[97] J. E. Demuth, D. Schmeisser, and Ph. Avouris. Resonance scattering of electrons from N2 , CO, O2 and H2 adsorbed on a silver surface. Phys. Rev. Lett.,
47(16):1166–1169, 1981.
[98] C. L. Allyn, T. Gustafsson, and E. W. Plummer. The orientation of CO adsorbed onNi(100). Chem. Phys. Lett., 1:127–132, 47.
[99] K. H. Frank, H. J. Sagner, E. E. Koch, and W. Eberhardt. 2π ∗ level of chemisorbed
CO on Ni(111): Nearest-neighbor interactions versus bonding to the substrate.
Phys. Rev. B, 38(12):8501, 1988.
[100] N. V. Richardson and A. M. Bradshaw. The frequencies and amplitudes of CO
vibrations at a metal surface from model cluster calculations. Surf. Sci., 88(1):225–
268, 1979.
[101] A. M. Lahee, J. P. Toennies, and Ch. Wöll. Low energy adsorbate vibrational modes
observed with inelastic helium atom scattering: CO on Pt(111). Surf. Sci., 117(13):371–388, 1986.
[102] H. Ibach and D. L. Mills, editors. Electron Energy Loss Spectroscopy and Surface
Vibrations. Academic Press, New York, 1982.
[103] A. Alavi, P. Hu, T. Deutsch, P. Silvestrelli, and J. Hutter. CO oxidation on Pt(111):
An ab initio density functional theory study. Phys. Rev. Lett., 80(16):3650–3683,
1998.
[104] B. Poelsema, L. K. Verheij, and G. Comsa. He-scattering investigation of CO migration on Pt(111). Phys. Rev. Lett., 49(23):1731–1735, 1982.
[105] B. N. J. Persson and R. Rydberg. Vibrational phase relaxation at surface: CO on
Ni(111). Phys. Rev. Lett., 54(19):2119–2122, 1985.
[106] B. E. Hayden and A. M. Bradshaw. The adsorption of CO on Pt(111) studied byinfrared reflection-adsorption spectroscopy. Surf. Sci., 125(3):787–802, 1983.

144[107] E. Schweizer, B. N. J. Persson, M. Tushaus, D. Hoge, and A. M. Bradshaw. The potential energy surface, vibrational phase relaxation and the order-disorder transitionin the adsorption system Pt(111)-CO. Surf. Sci., 213(1):49–89, 1989.
[108] Ch. Hess, S. Funk, M. Bonn, D. N. Denzler, M. Wolf, and G. Ertl. Femtosecond
dynamics of chemical reactions at surfaces. Appl. Phys. A, 71(5):477–483, 2000.
[109] P. Jakob and B. N. J. Perrson. Infrared spectroscopy of overtones and combination
bands. J. Chem. Phys., 109(19):8641–8651, 1998.
[110] C. Zhang, P. Hu, and A. Alavi. A density functional theory study of CO oxidation
on Ru(0001). J. Chem. Phys., 112(23):10564–10570, 2000.
[111] X-ray data booklet. Lawrence Berkeley National Laboratory, 2001.
[112] SES-100 and SES-200 electron spectrometer from Gammadata-Scienta, Sweden.
[113] J. Ullrich, R. Moshammer, R. Dörner, O. Jagutzki, V. Mergel, H. Schmidt-Böcking,and L. Spielberger. Recoil-ion momentum spectroscopy. J. Phys. B: At. Mol. Opt.
Phys., 30:2917–2974, 1997.
[114] 3-dimensional Multihit-MCP-Detector from RoentDek GmbH, Germany.
[115] R. Moshammer, M. Unverzagt, W. Schmitt, J. Ullrich, and H. Schmidt-Böcking.
A 4π recoil-ion electron momentum analyzer: a high-resolution “microscope” forthe investigation of the dynamics of atomic molecular and nuclear reactions. Nucl.
Instr. and Meth. in Phys. Res. B, 108:425–445, 1996.
[116] M. A. Abdallah, A. Landers, M. Singh, W. Wolff, H. E. Wolf, E. Y. Kamber,
M. Stöckli, and C. L. Cocke. Capture and ionization processes studied with
COLTRIMS. Nucl. Instr. and Meth. in Phys. Res. B, 154:73–82, 1999.
[117] G. J. Schulz. Resonances in electron-impact on diatomic-molecules. Rev. Mod.
Phys., 45(3):423–486, 1973.
[118] N. F. Lane. Theory of electron-molecule collisions. Rev. Mod. Phys., 52(1):29–119,
1980.
[119] M. Allan. Study of triple-states and short-lived negative-ions by means of electronimpact spectroscopy. J Elec. Spect. Relat. Phenom., 48(3-4):219–315, 1989.
[120] W. Domcke and L. S. Cederbaum. Simple formula for vibrational structure of resonances in electron-molecule scattering. J. Phys. B, 10(2):L47–L52, 1977.
[121] W. Domcke and L. S. Cederbaum. Vibration-induced narrowing of electronscattering resonances near threshold. J. Phys. B, 13(14):2829–2838, 1980.
[122] P. W. Atkins, editor. Physical Chemistry. W. H. Freeman Company, 2001.

145[123] A. Groß, editor. Theoretical Surface Science: A Microscopic Perspective. SpringerVerlog, Berlin, 2003.
[124] D. Menzel and R. Gomer. Desorption from metal surfaces by low-energy electrons.
J. Chem. Phys., 41(11):3311–3328, 1964.
[125] P. A. Redhead. Interaction of slow electrons with chemisorbed oxygen. Can. J.
Phys., 42:886–905, 1964.
[126] J. W. Gadzuk, L. J. Richter, S. A. Buntin, D. S. King, and R. R. Cavanagh.
Laser-excited hot-electron induced desorption: A theorectical model applied to
NO/Pt(111). Surf. Sci., 235(2-3):317–333, 1990.
[127] J. A. Misewich, T. F. Heinz, and D. M. Newns. Desorption induced by multiple
electronic transitions. Phys. Rev. Lett., 68(25):3737–3740, 1992.

ABSTRACT
Using ultrafast extreme-ultraviolet pulses for time-resolved dynamics of molecules
chemisorbed on metal surfacesby
Chi-Fong Lei

Chair: Henry C. Kapteyn

The time-resolved observation of chemical reactions on metal surfaces represents avery important step towards understanding how these surfaces work as catalysts. Developming techniques that would allow us to observe these dynamics is therefore very importantfor both scientific and industrial applications. However, experimental methods that combine both surface sensistivity and sub-picosecond time resolution have proven to be challenging. In this thesis, I describe a ”Time-Resolved extreme-Ultraviolet Photoemission
Spectroscope” (TR-UPS), that allows us to observe the dynamics of adsorbate moleculeson surfaces with femtosecond time resolution. This experimental tool combines state-ofthe-art techniques for generating EUV light (42 eV photon energy, sub-10 fs pulse width)using high-harmonic generation of intense ultrafast infrared pulses, with the technique ofphotoelectron spectroscopy.

Two experimental investigations have been performed with the TR-UPS. A saturation
layer of O2 adsorbed onto a Pt(111) single crystal surface at 77 K results in the oxygen
preferentially residing on the bridge-site of the platinum surface. By exciting the O 2 /Pt
system with an ultrafast pulse, we can then observe time-dependent changes in the electronic structure of the surface/adsorbate that corresponds to movement of the oxyen fromthe bridge-sites to the hollow-sites of the platinum surface. This change occurs in about
half a picosecond. Subsequently, the oxygen molecules relax back to their original configurations in under 5 ps. In the second experiment, CO molecules are introduced onto thePt(111) surface. The CO molecules are known to adsorb onto the platinum surface in an
upright position, parallel to the platinum surface normal. Using symmetry properties of
the CO molecular orbitals, I determine the oscillation period of the frustrated translationalmode of the CO molecules to be about 950 fs.