PHYSICAL REVIEW B 84, 155412 (2011)

Photostimulated desorption of Xe from Au(001) surfaces via transient Xe− formation
Akihiko Ikeda,* Masuaki Matsumoto, Shohei Ogura, Katsuyuki Fukutani,† and Tatsuo Okano‡
Institute of Industrial Science, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8505, Japan(Received 5 August 2011; revised manuscript received 14 September 2011; published 11 October 2011)
Photostimulated desorption (PSD) of Xe atoms from the Au(001) surface in thermal and nonthermal regimes
was investigated by the time-of-flight measurement at photon energies of 6.4 and 2.3 eV. Xe was desorbed ina thermal way at high laser fluence, which was in good agreement with theoretical simulations. At a low laser
fluence, on the other hand, desorption was induced only at a photon energy of 6.4 eV by a nonthermal one-photon
process. We argue that the nonthermal PSD occurs via transient formation of Xe− on Au(001). The lifetime of
Xe− is estimated to be ∼15 fs with a classical model calculation. Whereas the electron affinity of Xe is negativein the isolated state, it is stabilized by the metal proximity effect.
DOI: 10.1103/PhysRevB.84.155412

PACS number(s): 68.43.Tj, 32.10.Hq, 68.43.Vx, 79.20.La

I. INTRODUCTION

Photostimulated processes at solid surfaces have been a
topic of extensive studies because they allow us to control
adsorbates in either thermal or nonthermal ways.1–3 Laserinduced thermal desorption (LITD) was investigated in detailfor the systems of CO/Fe(110) (Ref. 4) and Xe/Cu,5 and has
been successfully applied to the studies of surface diffusion
combined with low-energy electron microscopy6 or withscanning tunneling microscopy.7 Nonthermal photostimulated
phenomena, on the other hand, provide us with pathways thatare nonaccessible in a thermal process. The nonthermal photostimulated desorption (PSD) of rare gas atoms from metal
surfaces has been investigated using photons of two energy
regions: At hν > 7 eV, the excitonic or ionic excitation of themono- and multilayers of Ar and Kr induces desorption,8 while
infrared light at hν < 1 eV causes the direct excitation of the vibrational mode in the physisorption well to a continuum state.9
The nonthermal PSD of Xe/metal using 1–7 eV photons
has been considered not to occur. Generally, the mechanismof nonthermal PSD using photons of 1–7 eV is understood interms of formation of the transient negative ion (TNI)10 and theAntoniewicz model,11 where a substrate conduction electron
is photoexcited to the adsorbate affinity level. The ground state
Xe in the gas phase does not bind an electron stably,12–15 which
has been confirmed both theoretically and experimentally withthe exception of Ref. 16. Xe atoms physisorb on a metal
surface. Physisorption is assumed to occur with little influenceon the electronic states. Hence, it has been anticipated that the
PSD of Xe/metal via TNI is absent. Condensed Xe, however,has been reported to have modified electronic states comparedwith the isolated ones due to hybridization with the orbitals ofneighboring atoms. It is known that it takes 0.5 eV to remove
an excess electron from the bulk Xe,17 and also that the groundstate XeN clusters with N > 6 stably bind an electron,16
indicating that the electron affinity level of Xe is shifted
downward or broadened depending on the phase of Xe. In thissense, adsorption of Xe onto metal surfaces may well result ina shift and/or broadening or even narrowing of its affinity levelby hybridization of the unoccupied orbitals with the substrateelectronic states, as is predicted by theoretical studies.18,19
In the present paper, we report an experimental study of
LITD and nonthermal PSD of Xe from a Au(001) surface at
1098-0121/2011/84(15)/155412(5)photon energies of 6.4 and 2.3 eV. At a high laser fluence,
Xe desorption was thermally induced at both photon energies,
which is in good agreement with theoretical calculations. At
a low laser fluence, on the other hand, Xe desorption was
induced nonthermally by 6.4 eV photons as a one-photon
process, whereas little desorption was observed with 2.3 eV
photons. We argue that the nonthermal PSD proceeds with atransient formation of Xe− as a result of the photoexcitation ofsubstrate conduction electrons. A classical model calculationof Xe desorption reproduces both the experimentally observed
TOF and nonthermal PSD cross section, assuming a value ofthe Xe− lifetime to be ∼15 fs.
II. EXPERIMENT

A single-crystal disk of Au(001) was mounted on a coldhead after being chemically and mechanically polished. The
Au(001) surface was cleaned by several cycles of Ar+ ion sputtering at 0.5 keV and annealing at 700 K in an ultrahigh vacuum
chamber (p = 2.0 × 10−8 Pa). The cleanliness of Au(001) was
confirmed by observing the (5 × 20) reconstructed patternwith LEED (Ref. 20) and no contamination in AES. A Xe
monolayer was formed by dosing Xe gas of 3 L (1 L =1.33 × 10−4 Pa s) to the sample surface at 23 K (Ref. 21)
cooled by a closed-cycle He-compression-type refrigerator.
The ArF excimer laser (6.4 eV, 8 ns) and the second harmonics
of Nd:YAG (yttrium aluminum garnet) laser (2.3 eV, 7 ns)
pulses were guided onto the area of 1 mm2 on the sample
surface with an incidence angle of 25◦ from the surface normal.
The desorbing Xe atoms were detected by a quadrupole mass
spectrometer (QMS) located in the surface normal direction
with a flight distance of 10 cm. The output signal was amplified
by a fast current amplifier and recorded with an oscilloscopesynchronized with the laser pulse. The QMS was operated in a
low-resolution mode, which enabled high-sensitivity detectionof Xe atoms.
III. RESULTS

The time-of-flight (TOF) of desorbing Xe atoms upon laser
irradiation was recorded at a wide range of laser pulse energy
absorbed by the sample (IL ) for both 6.4 and 2.3 eV photons.
IL was estimated by taking account of the reflectivity on

155412-1©2011 American Physical Society

IKEDA, MATSUMOTO, OGURA, FUKUTANI, AND OKANO

(a)

400

(a) 6.4 eV, 67 mJ/pulse·cm2

0.66.4 eV
2.3 eV300(b) 2.3 eV, 67 mJ/pulse·cm2

0.4(c) 6.4 eV, 4.8 mJ/pulse·cm

0.2

TD1 (K)

Normalized Intensity

PHYSICAL REVIEW B 84, 155412 (2011)

400
300
200
100
0
0

200

2

100
2

(d) 2.3 eV, 5.1 mJ/pulse·cm
0.0

0
1.0

2.0

3.0

Time of Flight (ms)FIG. 1. Time-of-flight spectra of desorbing Xe atoms from
Au(001) following pulse laser irradiation. On each spectra, the photon
energy and the absorbed energy by the sample are denoted. Dataof [(a),(b)] and [(c),(d)] were recorded with a single pulse and theaccumulation of over 120 pulses, respectively.

Au (0.8 for 2.3 eV and 0.2 for 6.4 eV). Figure 1 shows
typical TOF results. The data reveal a maximum at a TOF
of 400 μs with a tailing feature in the long TOF region. TOF
was recorded with only one pulse for both 6.4 and 2.3 eV
photons with IL > 10 mJ/pulse cm2 [Figs. 1(a) and 1(b)]. WithIL < 10 mJ/pulse cm2 , on the other hand, TOF was recordedby accumulating over 120 data [Figs. 1(c) and 1(d)] because
the Xe desorption yield was small. Whereas a substantial
desorption yield was observed with 6.4 eV photons, as shownin Fig. 1(c), no significant signal was recorded with 2.3 eV
photons, as shown in Fig. 1(d). Solid curves in Fig. 1 are fits tothe data with a sum of two Maxwell-Boltzmann (MB) velocity
distributions described as f (v) = A1 v 2 exp(−mv 2 /2kTD1 ) +A2 v 2 exp(−mv 2 /2kTD2 ), where m and k are mass of a Xe
atom and the Boltzmann constant, respectively, and Ai andthe translational temperature TDi are fitting parameters. In theanalysis, the form is converted to the flux weighted form.
In the following, we discuss only the fast component of
the TOF.
Figures 2(a) and 2(b) show TD1 and the Xe desorption yieldof the first component (Y1 ) plotted as a function of IL , respectively. We first focus on the region of IL > 32 mJ/pulse cm2 .
In this region, TD1 increases with increasing IL from about
200 K at 32 mJ/pulse cm2 and saturates at about 300 K,
for both 6.4 and 2.3 eV photons. In Fig. 2(b), Y1 shows asharp increase at 32 mJ/pulse cm2 . The behavior of TD1 andY1 indicates that the Xe desorption is thermally activated with
IL > 32 mJ/pulse cm2 .4,5 For the quantitative analysis, we carried out numerical calculations of the surface temperature (TS )during laser irradiation on the basis of the one-dimensional
heat conduction equation.22 Subsequently, the time evolutionof the Xe coverage and the Xe desorption rate (i.e., LITD)
was deduced by employing the first-order desorption kinetics
assuming the activation energy for desorption of Xe from
Au(001) to be 240 meV.23 By the calculations above, we
obtained the surface temperature at maximum Xe desorption

Y1 (arb. units)

0.0

50

0

50

100

1

0
0

0

10

5

10

(b)

20
40
60
2
Absorbed laser pulse energy I L (mJ/pulse·cm )FIG. 2. (a) Translational temperature (TD1 ) of Xe atoms desorbed
from Au(001) following laser irradiation as a function of absorbed
laser pulse energy (IL ). The solid and dashed lines are calculated
results of the surface temperature at the maximum desorption rate
(TDM ) and the maximum surface temperature (TSM ), respectively. The
inset is a magnification of the small IL region. (b) Desorption yieldof Xe atoms (Y1 ) following the laser irradiations with 6.4 eV photons
as a function of IL . Calculated results of Y1 are shown by the solid
curve. The inset is a magnification of the small IL region.

rate (TDM ) and the maximum surface temperature (TSM )following laser pulse irradiation with IL .
TDM and TSM deduced from the calculation are depicted
as a function of IL in Fig. 2(a) as dashed and solid curves,
respectively. TD1 obtained by the experimental results is ingood agreement with TDM , indicating that LITD of Xe isdominant with IL > 32 mJ/pulse cm2 . TDM deviates from TSM
with IL > 50 mJ/pulse cm2 because desorption occurs before
the surface temperature reaches its maximum.4 The solid line
in Fig. 2(b) shows a calculated result of the Y1 by LITD
as a function of IL . The calculated Y1 sharply increases at
35 mJ/pulse cm2 and saturates above 55 mJ/pulse cm2 . This
is in good agreement with the experimental data that thedesorption yield exhibits a steep increase at 32 mJ/pulse cm2 .
This thresholdlike behavior is typical of LITD. However, theexperimental data in Fig. 2(b) monotonously increases incontrast to the saturating behavior of the calculated curve,
which may be caused by either spatial inhomogeneity ofthe desorption laser intensity24 or coverage dependence of
the activation energy for desorption due to the attractive
interactions between the adsorbates.4
We turn next to the region of IL < 24 mJ/pulse cm2 , where
desorption of Xe is observed only at 6.4 eV. As can be seenin Fig. 2(a), TD1 significantly deviates from the calculated
result of TSM in this region of IL . The inset in Fig. 2(a) shows

155412-2

PHOTOSTIMULATED DESORPTION OF Xe FROM Au(001) . . .

2.3 eV
EF

200

Au

0

E Affi (Xe-)
Xe

2|ψXe |-200
VMP: Xe/Au(001)

0.6

2.5

3.0
3.5
z (Angstrom)

(b)
(3)

(2)

0.2

4.0

(1) 15 fs(2) 10 fs(3) 5 fs

0.4
(1)

0.0
0.0

0.5
1.0
1.5
Time of Flight (ms)

800 (c)
600
400
200
0
0 10 20 30
Lifetime τ (fs)2

Potential Energy (meV)

6.4 eVE Vac

σ PSD (cm )We first discuss the initial excitation of the nonthermal PSD
of Xe from Au(001) upon irradiation of 6.4 eV photons. As
the initial excitation, we argue that the negative ion state of Xe
is formed via the photoexcitation of the substrate electron.10
Other excitation pathways can be excluded for the following
reasons. The first-excitation energy of Xe from the ground state(5s 2 5p6 ) to the metastable state (5s 2 5p5 6s) is 8.3 eV.8 When
Xe is condensed into a two-dimensional layer on a surface, theexcitation energy might be modified, as denoted by the surface
exciton. The value is, however, reported to be little modified
in the monolayer adsorption regime,25 suggesting that such
excitation is unlikely to occur at 6.4 eV. The first ionizationenergy is 12.1 eV,26,27 which is also unreachable with 6.4 eV
photons even though it is reduced due to the image charge
effect by ∼2.9 eV.25 Xe desorption from Ag nanoparticles(AgNP) or Si(001), on the other hand, is also reported tooccur via surface-plasmon excitation of AgNP at 2.3– 4.0 eV
photons28 and localized surface phonon excitation of Si(001) at
1.1–6.4 eV photons.29 Desorption via direct excitation from thebound state to a continuum state took place at a photon energyof lower than 1 eV.9,30 All these desorption mechanisms canbe ruled out because the nonthermal PSD of Xe was observed
only at 6.4 eV and not at 2.3 eV photoirradiation.
The work function of the Au(001) surface is 5.0 eV, whichis reduced by ∼0.5 eV with Xe adsorption. Therefore, theelectronic states nearby the vacuum level are accessible withthe hot electrons from the substrate band created by 6.4 eV
photoexcitation, and not by 2.3 eV as schematically shown in
Fig. 3(a). Although the electron affinity of Xe atoms in the gas
phase is known to be negative,12 the following studies suggest
stabilization of the affinity level due to Xe condensation. Bulk
Xe has a conduction band minimum (CBM) at 0.5 eV below
the vacuum level.17,31 Haberland et al. found that the groundstate XeN clusters are able to bind an electron stably with
N > 6,16 of which the electron affinity is calculated to bea few meV.32,33 These studies suggest that interaction withneighboring atoms lowers the electron affinity level of Xe dueto the mixing between the unoccupied orbitals. Furthermore,
the image charge effect on metal surfaces shifts the electron
affinity level downward by ∼1.0 eV.

VIP: Xe /Au (001)

(a)

-400
Intensity (arb. units)

IV. DISCUSSION-

TDC (K)a magnification of TD1 with IL < 10 mJ/pulse cm2 . In thisregion, TD1 is independent of IL and constant at 300 ± 20 K,
which is much higher than TSM (<100 K). Since in the region of
IL < 24 mJ/pulse cm2 the calculated result of LITD fails to account for the experimental data, other desorption mechanisms
should be operative. Especially with IL < 10 mJ/pulse cm2 ,the LITD yield of Xe is negligible because TSM is too low
for the thermal activation of Xe desorption. Therefore, only
nonthermal PSD of Xe atoms from Au(001) is operative inthis region of IL . As shown in the inset of Y1 as a function of
IL in Fig. 2(b), Y1 with 6.4 eV photons linearly increases withincreasing IL , indicating that the observed nonthermal PSD
is a one-photon process. The nonthermal PSD cross section
σPSD was deduced to be 10−21 –10−22 cm2 by comparing
the nonthermal PSD yield with the LITD yield of the Xemonolayer.

PHYSICAL REVIEW B 84, 155412 (2011)

-20

10

2.0

(d)

-22

10

-24

10

10 20 30
Lifetime τ (fs)FIG. 3. (a) Adiabatic potential of Xe and Xe− on Au(001) anda schematic of the Antoniewicz model of neutral desorption used
for the model calculations. (b) Calculated results of time-of-flight(TOF) of desorbing Xe for several Xe− lifetimes. (c) Translationaltemperature (TDC ) of the calculated TOF obtained by analyzing witha Maxwell-Boltzmann distribution as a function of Xe− lifetime.
(d) Calculated result of the nonthermal PSD cross section as a functionof Xe− lifetime.

Assuming that the excitation intermediate is the negative
ion state, a plausible desorption mechanism is the Antoniewicz
model.11 In the model, for the appreciable desorption to occur,
the lifetime of the Xe− state is required to be long enough.
We tentatively estimated the lifetime (τ ) of Xe− on Au(001)that reproduces the experimentally observed values of TD1
and σPSD . σPSD is a product of the photoionization crosssection σPI and desorption probability PD . We assume, as afirst approximation, that the σPI is as large as ∼10−16 cm2 .27
TOF of desorbing Xe and PD are calculated on the basis of
the Antoniewicz model and classical kinetics, as is depicted
in Fig. 3(a). Initially, Xe atoms are trapped at the bottom of
the physisorption well described by a Morse potential of theform VMP (z) = D[1 − exp{−α(z − z0 )}]2 , where D, α, and z0
represent the depth (240 meV),23 the width (14 nm−1 ), andthe position (3.0 Å),19 respectively. The distribution of theinitial position of Xe is accounted for as described in Ref. 27.
Upon Xe− formation, the adiabatic potential of the Xe atom
evolves into the form VIon (z) = VMP (z) + VIP (z) + E, where
VIP (z) = −e2 /(16π 0 z) is the image charge potential and E
is an excitation energy. Due to the image charge attraction, the155412-3

IKEDA, MATSUMOTO, OGURA, FUKUTANI, AND OKANOPHYSICAL REVIEW B 84, 155412 (2011)

Xe atom is first attracted toward the surface, and is neutralized
at a certain distance from the surface. The nuclear motion
on VIon (z) is treated classically. If the Xe atom gains enoughenergy, it escapes the physisorption well leading to desorption.
We assume that the neutralization rate of Xe− is described byR(t) = exp(−t/τ )/τ independent of z.
Figure 3(b) shows the TOF results of Xe calculated forlifetimes of 5–15 fs. Each TOF is well expressed by a single
MB velocity distribution with a translational temperature
TDC . Figure 3(c) shows the obtained TDC of the calculated
TOF as a function of τ , where τ = ∼13 fs reproduces
the experimentally observed TD1 of 300 ± 20 K. Figure 3(d)shows the calculated result of σPSD as a function of τ , whereτ = 17 ± 5 fs reproduces the experimentally observed σPSD
of 10−21 –10−22 cm2 . It is worth emphasizing that the two
experimental data of TD1 and σPSD are well reproduced by acommon τ value of ∼15 fs based on the Antoniewicz model.
The fact is indicative of the validity of the present model.
Walkup et al. have shown that the classical adiabatic potential
concerning the image charge potential is essentially correct,
although it is slightly different from the one obtained by aquantum-mechanical treatment. They have furthermore shownthat the classical treatment of nuclear motion is valid as long as
distribution of the initial Xe position is accounted for and that it
qualitatively reproduces the kinetic-energy distribution.34 It isnoted that the affinity level of Xe should lie below the vacuum
level for the τ to be as long as 15 fs.
The obtained value of τ ∼15 fs corresponds to the linewidthof 70–120 meV for the Xe− state. Padowitz et al. found that inusing the two-photon photoemission spectroscopy, the image
charge state on clean Ag(111) is shifted by Xe adsorption
due to the coupling with the Xe orbitals.35,36 The linewidth
obtained in the present study is similar to the value of 25–
50 meV observed for the image charge states (n = 1,2,3) on
Xe/Ag(111) at 0.6–0.1 eV below the vacuum level. Hence, wesuggest the image charge state of Xe/Au(001) is resonancedwith the affinity level of Xe, which causes the PSD. We notethat a smaller estimation of σPI ∼ 10−18 cm2 in the model
calculation results in a linewidth of ∼35 meV.
As already mentioned above, two possible mechanisms of
Xe− stabilization are hybridization of unoccupied orbitals andthe image charge effect. Since unoccupied orbitals have an
extended feature compared with occupied orbitals, unoccupied
states could be appreciably hybridized with substrate stateseven in a weakly bound physisorption well. In addition to these
two factors, we discuss another possible reason for the Xe−
formation on a metal surface. In the gas phase, contrary to theground state Xe (5s 2 5p6 ), metastable Xe∗ (5s 2 5p5 6s) binds anelectron to form a transient Xe− (5s 2 5p5 6s 2 ) with a large crosssection (∼10−16 cm2 ).37 In the gas phase, the 5s 2 5p5 6s stateis located at 8.3 eV above the ground state. A recent density
functional study19 has shown that the Xe adsorption on a metal
surface results in a partial depletion of the occupied Xe 5pz
state and a partial occupation of the previously unoccupied Xe
6s and 5d states. This indicates mixing of the 5s 2 5p5 6s state
upon adsorption on a metal surface, which may contribute to
the stabilization of the Xe− state.
Lastly, we comment on the result of an earlier study on
the nonthermal PSD of Xe from Ru(001) surfaces.8 In the
study, no significant desorption was observed from Xe monoand multilayers following 7–30 eV photoirradiations, whereas
desorption from Ar mono- and multilayers and Kr multilayers
were observed. As discussed in the present paper, Xe− is
expected to be formed following the photoirradiations of hν >6 eV, and subsequently Xe desorption is expected to occur. Although the desorption cross section is not mentioned in Ref. 8,
we suspect σPSD of ∼10−22 cm2 was too small for the signal
to be detected in their experimental condition. Arakawa et al.
reported that the absolute yield of the PSD from solid Ar following 12–50 eV photons is as large as ∼0.1 atoms/photon,38
indicating that the cross section of the Xe PSD via the Xe−
formation observed in the present study is several orders ofmagnitude smaller than those of Ar via exciton excitation.
V. CONCLUSION

In conclusion, we have investigated the PSD of Xe
on Au(001) at photon energies of 2.3 and 6.4 eV. Withdecreasing pump laser fluence, the desorption was found toundergo transition from thermal to nonthermal regimes. The
nonthermal PSD of Xe occurred only at 6.4 eV as a one-photon
process, and the desorption proceeds via the Antoniewicz
model with transient negative ion formation. On the basisof the model calculation, the lifetime of Xe− is estimatedto be ∼15 fs. These results strongly suggest that the affinity
level of Xe is substantially stabilized by the metal proximity
effect.
ACKNOWLEDGMENTS

This work was supported by Grant-in Aid for Scientific
Research (A) of Japan Society for the Promotion of Science(JSPS) and by the Sasakawa Scientific Research Grant fromthe Japan Science Society. A.I. acknowledges support froma Research Assistant of the Global Centre of Excellence forPhysical Science Frontier of Tokyo University, Japan.

*

2

†

3

a-ikeda@iis.u-tokyo.ac.jp
fukutani@iis.u-tokyo.ac.jp‡
okano@iis.u-tokyo.ac.jp
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