© 2014 The Chemical Society of Japan

Bull. Chem. Soc. Jpn. Vol. 87, No. 3, 341­354 (2014)

341

Award Accounts

The Chemical Society of Japan Award for Creative Work for 2012

Nonadiabatic Electronic Dynamics in Isolated Molecules
and in Solution Studied by Ultrafast Time­Energy Mappingof Photoelectron Distributions
Toshinori Suzuki1,2,3
1

Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502

2

CREST, Japan Science and Technology Agency, Tokyo 102-0076
RIKEN Center for Advanced Photonics, RIKEN, Wako, Saitama 351-0198

3

Received October 4, 2013; E-mail: suzuki@kuchem.kyoto-u.ac.jp

Nonadiabatic electronic dynamics contribute to the diversity of chemical reactions. We investigate nonadiabatic
dynamics in isolated molecules and aqueous solutions by time-resolved photoelectron spectroscopy. For isolated
molecules in the gas phase, we combine two-dimensional imaging detection of electrons and a sub-20 fs deep UV andvacuum UV light source to measure time-evolution of the photoelectron kinetic energy and angular distributions. Time­energy mapping of photoelectron angular anisotropy reveals the S2 ¼ S1 internal conversion dynamics through conicalintersection in pyrazine, benzene, and toluene. The time­energy mapping is also employed to extract the photoelectron
angular distribution in the molecular frame for nitric oxide. For electronic dynamics in aqueous solution, we employ timeresolved photoelectron spectroscopy using a liquid beam and an electrostatic or a time-of-ﬂight electron energy analyzer.
Successful observation of ultrafast electron-transfer reactions in aqueous solutions and measurement of electron binding
energies of solvated species suggest promising future developments of this very young ﬁeld.

1. Introduction
The structures and dynamics of molecules are determined byquantum mechanics, while the quantum mechanical equation of
motions of the nuclei and electrons is not exactly solvable. Born
and Oppenheimer have approximated the rigorous equation
by separating it into a set of equations of the nuclei and of theelectrons, which facilitated a quantum mechanical descriptionof chemistry.1 Based on the approximation, a chemical reactionis understood as quantum-mechanical nuclear motion on an
adiabatic potential energy surface created by electronic motions.
The approximation, however, breaks down at critical nuclear
conﬁgurations where multiple electronic states have similarenergies. Consequently, nonadiabatic transitions (or hoppings)
occur between diﬀerent potential energy surfaces in thevicinities of these critical conﬁgurations. The nonadiabatic
transitions change reaction pathways and product yields,
contributing to the diversity of chemical reactions.
Nonadiabatic transitions are ubiquitous in the excited state
dynamics of polyatomic molecules, because polyatomic molecules have a large number of excited electronic states withina narrow energy range, which cause (avoided) crossings ofpotential energy surfaces. The conical intersection is one of themost important topographic features of the surface crossingsfor nonadiabatic transitions.2­5 Figure 1 shows photochemical
reaction pathways mediated by conical intersections of thepotential energy surfaces in benzene.6 The characteristic funnel
shape of the conical intersection facilitates eﬃcient drainingof a nuclear wave packet from an upper to a lower potential
energy surface. Although remarkable progress has been madein our understanding of chemical reactions in the past 30 years,
nonadiabatic dynamics of polyatomic molecules remain highly
challenging subjects of research. Direct experimental observation of ultrafast electronic dynamics and interplay with theoretical calculations are indispensable in this research ﬁeld.
Photoelectron (photoemission) spectroscopy is an experimental method based on the photoelectric eﬀect, and it was
initiated in the late 1950s to early 1960s.7­9 One of the pioneersof photoelectron spectroscopy, Kai Siegbahn, received theNobel Prize of Physics in 1981.10 Photoelectron spectroscopy
induces emission of an electron from a material using a photon
beam (such as UV or X-ray radiation) and measures the kinetic
energies of photoelectrons. Time-resolved photoelectron spectroscopy (TRPES) is an advanced form of photoelectron spectroscopy using a pair of laser pulses.11­15 The pump pulse
creates a wave packet in an electronically excited state, and theprobe pulse interrogates its time-evolution by photoemission.
TRPES requires the laser pulse durations to be shorter than the

Published on the web March 15, 2014; doi:10.1246/bcsj.20130272

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wetted with a sample solution were developed for this purpose.16 A high-speed liquid microjet reduced the diﬃculty,
since a small diameter (ca. 10­20 ¯m) minimizes the surface
area from which solvents evaporate, and a high speed minimizes the temperature drop in the transport from the nozzle tothe ionization point.17 The temperature of water jet at 1 mm
downstream of the nozzle is estimated to be 5­10 °C.18 We
devoted our own eﬀorts to develop TRPES of liquids for a
decade and succeeded for the ﬁrst time in 2010.19,20
This article is organized as follows. In Section 2, I describe
TRPES of gas-phase reactions using two-dimensional photoelectron imaging and a novel ultrafast laser in the deep UV(DUV) and vacuum UV (VUV) regions. The key technique istime­energy mapping of photoelectron angular anisotropy,
which provides information on electron conﬁguration of thenonstationary state. We examine ultrafast internal conversion
in isolated molecules of pyrazine, benzene, and toluene anddiscuss their diﬀerences. In Section 3, I describe extraction of
the molecular-frame photoelectron angular distribution, seen byan observer on a molecule, using the time­energy mapping ofphotoelectron angular anisotropy. Section 4 is devoted to the
new research area of TRPES of liquids. An electron-transfer
reaction in water and generation of a hydrated electron are
discussed. Section 5 presents the summary and perspectives for
future studies.
Figure 1. Photochemical reactions of benzene from S2 andS1 states mediated by conical intersections. Reproducedwith permission from Ref. 6.

time scales of the dynamics of interest and the pulse energies
suﬃciently high to induce ionization within the pulse durations. A highly sensitive electron detection method is also
important for TRPES, because the number of photoelectrons
generated per optical pulse-pair must be minimized to avoid
electrostatic repulsion between the photoelectrons, which
would otherwise alter the electron kinetic energies.
Ionization has several important advantages as a probing
method of chemical reactions. First of all, the ﬁnal state isenergetically continuous, so that ionization can be induced
from any part of the excited state potential energy surfaces aslong as the photon energy is suﬃcient. Second, the ionization
continuum is degenerate for diﬀerent symmetries and spinstates of photoelectrons, so that ionization is almost always anallowed transition; the singlet and triplet excited states areequally observed. Third, high sensitivity can be obtained bycollecting photoelectrons using electromagnetic ﬁelds.
We employ TRPES of gas-phase reactions to elucidate fundamental aspects of electronic dynamics in elementary chemical reactions. The dynamics, however, are signiﬁcantly alteredin solution, especially in polar protic solvents such as water.
While solvent eﬀects have been employed for centuries tocontrol solution chemistry, the coupled electronic and solvation
dynamics in solution are not well understood. Therefore, it
is highly interesting to develop TRPES of liquids to tackle thisclassic problem in chemistry. Since the 1970s, there has beena long history of endeavors to bring volatile liquids into highvacuum photoelectron spectrometers.16 For an example, intriguing devices such as a rotating wire and a rotating trundle

2. Ultrafast Internal Conversion in Isolated Aromatic
Molecules Studied by Time-Resolved Photoelectron
Imaging Using a Sub-20 fs DUV/VUV Light Source
2.1 Experimental Technique. When photoelectron spectroscopy was initiated in the 1950s using continuous lightsources, experiments were performed using electrostatic electron energy analyzers.7­9 Although these analyzers are stillprincipal instruments in high-resolution X-ray photoelectron
spectroscopy today, relatively long integration times of thesignals are required owing to small solid angles and narrowenergy windows. With the development of intense pulsed(nanosecond and picosecond) lasers, photoelectron spectroscopy using multiphoton ionization started in the 1980s, which
employed time-of-ﬂight (TOF) photoelectron spectrometers.
The TOF spectrometer measures ﬂight times of photoelectrons
from a sample to a detector, and it enables measurement of theentire photoelectron energy spectrum on a shot-to-shot basis.
Furthermore, a magnetic bottle was introduced to achieve anelectron collection eﬃciency of 50% with the TOF analyzer,
which enabled the application of photoelectron spectroscopy to
low-density species such as molecular and metal clusters insupersonic beams.21 However, a photoelectron angular distribution can hardly be measured with a magnetic bottle, as it
utilizes electron cyclotron motions in a magnetic ﬁeld.
Chandler and Houston invented an ion imaging techniquein 1988, in which ions emitted from a small target volume
in a Wiley-McLaren TOF mass spectrometer are accelerated
and projected onto a two-dimensional (2D) position-sensitive
detector.22 Eppink and Parker have modiﬁed the ion optic
electrodes and enabled 2D space focusing of ions to improve
the imaging resolution.23 Their method is now called velocity
map imaging, because the arrival position of the ion on thedetector plane is proportional to the velocity perpendicular

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Figure 2. Time-resolved photoelectron imaging apparatus.

to the ﬂight axis and independent of the ionization point.
The performance of acceleration electrodes (aberration etc.)
can be further improved by increasing number of electrodes
beyond the Eppink­Parker minimal design which uses threeelectrodes.24,25
We combined the femtosecond pump­probe method and2D imaging technique to initiate time-resolved photoelectron
imaging (TRPEI) in 1999.26,27 Figure 2 shows a schematic
diagram of TRPEI.28­30 An ultracold gas of target molecules iscreated by adiabatic gas expansion into vacuum, and the gas jet
is skimmed to create a supersonic molecular beam 2 mm indiameter. The beam is introduced into a photoelectron spectrometer and crossed with the pump and probe laser beams. The
pump pulse excites molecules to excited electronic states, andthe probe pulse induces photoemission to create an expanding
sphere of a photoelectron distribution. The photoelectrons
are accelerated in a static electric ﬁeld and projected onto a
position-sensitive detector. The detector consists of microchannel plates, a phosphor screen, and a CCD (or CMOS)
camera, and it records the arrival positions of the photoelectrons on the detector plane. Since both the pump and probe
laser polarizations are parallel to each other and to the detector
face, the original 3D distribution has axial symmetry aroundthe polarization direction. With this symmetry, the 3D distribution can be reconstructed from the projection image. The
advantage of TRPEI over other methodologies is the ability tomeasure time-evolution of the photoelectron angular distribution (PAD) with high eﬃciency and accuracy. We developed a2D electron-counting apparatus using a CMOS camera andreal-time centroiding calculations on a ﬁeld programmable gatearray circuit, which captured electron images at 1 kHz witha uniform sensitivity over the detector area.31 The imaging
detector is able to record simultaneous arrivals of many electrons, which is an advantage over other 2D position-sensitive
detectors such as delay-line detectors.
The photoelectron ejection angle is an important observable in photoelectron spectroscopy. Since the initial ensembleof molecules has an isotropic molecular axis distribution in
the absence of an external ﬁeld, it is an isotropic target forphotoionization, similar to the 1s orbital of a hydrogen atom.
Therefore, the anisotropy of the total physical system (molecule + radiation) prior to photoionization is caused by polarized photons. This anisotropy is transferred to the PAD after
photoionization. For linear polarization of the pump and probe
pulses parallel to each other, the photoelectron kinetic energy
and angular distribution in [1 + 1¤] photoionization is expressed as follows (the prime means diﬀerent color):
Iðt; E; ªÞ ¼·ðt; EÞ
f1 þ ¢2 ðt; EÞP2 ðcos ªÞ
4³
þ ¢4 ðt; EÞP4 ðcos ªÞgð1Þ

where t, ª, and E are the pump­probe time delay, the electron
ejection angle from the laser polarization direction, and thephotoelectron kinetic energy. Pn(x) are the n-th order Legendre
polynomials. ·(t,E) represents a photoelectron kinetic energy
distribution or photoelectron spectrum. ¢2(t,E) and ¢4(t,E)
are called anisotropy parameters, and the three scalar quantities ·(t,E), ¢2(t,E), and ¢4(t,E) in eq 1 are the observables in[1 + 1¤] TRPEI of gaseous samples.
Now we consider laser requirements in TRPEI experimentsby considering the most fundamental aromatic molecule ofbenzene. Its ionization energy is 9.24 eV, so that one-photon
ionization of benzene requires an ionization laser wavelength
shorter than 134 nm, and the pump­probe TRPES requires
at least one of the pump or probe wavelengths to be shorter
than 268 nm. One of the characteristic molecular vibrations ofbenzene is a totally symmetric ring-breathing mode with avibrational period of ca. 50 fs. In order to create a spatiallylocalized wave packet and observe its time evolution, thepump and probe pulses should be sub-30 fs. While tunable
DUV pulses down to ca. 200 nm can be generated using opticalparametric ampliﬁers and nonlinear optical crystals, a VUVpulse can hardly be generated by these methods. The pulse
durations are also generally longer than 30 fs. Thus, we devel-

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Figure 3. (a) Schematic diagram of the experimental setup. (b) Schematic drawing of the diﬀerential pumping system.
All dimensions are given in millimeters.

oped a ﬁlamentation four-wave mixing method32­34 in raregas to generate sub-20 fs DUV and VUV pulses (Figure 3).
Filamentation is a unique propagation scheme of an intenselaser pulse through a medium. An intense laser pulse alters
the refractive index of the medium (optical Kerr eﬀect), which
induces self-focusing of the pulse; however, as the focusedlaser pulse induces (tunnel-)ionization of the medium, theionized species create an opposite spatial gradient of the refractive index. Consequently, the laser pulse propagates throughthe medium as a tightly focused beam for a distance longer thanthe Rayleigh length,35 which facilitates eﬃcient wavelength
conversion in low density gases.
In our experiment, the fundamental (½: 775 nm, 25 fs) andthe second harmonic (2½: 390 nm, 30 fs) of a Ti:sapphire laser
are gently focused into a neon gas inducing the ﬁlamentation
four-wave mixing process of 2½ + 2½ ¹ ½ ¼ 3½ (264 nm).
Then, the cascaded four-wave mixing processes generate 4½(198 nm), 5½ (159 nm), and 6½ (134 nm) pulses simultaneously. The DUV and VUV pulses are transmitted through apinhole placed at the end of the neon gas cell. The central andperipheral parts of the output beam are separated spatially,and diﬀerent harmonics are isolated using dichroic mirrors.
The two diﬀerent harmonics are independently focused onto a
molecular beam with two concave mirrors. The cross-correlation between 4½ and 5½ is 18 « 2 fs.
2.2 S2 ¼ S1 Internal Conversion via a Conical Intersection in Pyrazine. Pyrazine was one of the ﬁrst molecules
studied with TRPES in 1999.27 The S2(1B2u, ππ*) ¼ S1(1B3u,nπ*) internal conversion of pyrazine (C4H4N2, D2h) is the bestknown example of ultrafast electronic deactivation mediatedby a conical intersection. The low-lying conical intersection
between the S2 and S1 potential energy surfaces of pyrazine
was identiﬁed in 1988.36 Thereafter, a number of theoreticalstudies have been performed on this system.37­50 Although

Figure 4. UV photoabsorption spectra of S1, S2, and S3 ofpyrazine-h4 (thin solid line) and pyrazine-d4 (thin dashedline) at room temperature. The spectra of our pump(264 nm, 4.70 eV) and probe (198 nm, 6.26 eV) pulses arealso shown in solid lines.

pyrazine has 24 normal modes, only a single mode Q10a (b1g)
mediates the S2­S1 coupling, and a few totally symmetric (ag)modes play principal roles in the vibrational dynamics. Seeland Domcke proposed TRPES of pyrazine theoretically in1991 using model calculations taking into account three vibrational coordinates of Q1, Q6a, and Q10a.51 Theoretical studies
have predicted that the internal conversion occurs within
30 fs, while the time resolution of our experiment in 1999
was 450 fs.26,27 Thus, we constructed a sub-20 fs DUV laserusing ﬁlamentation four-wave mixing and revisited the S2­S1
dynamics of pyrazine in 2010.52
Figure 4 shows the UV photoabsorption spectra of pyrazine(pyrazine-h4) and fully deuterated pyrazine (pyrazine-d4) vapor
measured at room temperature. Overlaid are the spectra of our
3½ (264 nm: blue) and 4½ (198 nm: red) pulses. The 3½ pulse
excites pyrazine to S2 near the origin and the 4½ pulse ionizes
from S2 and S1. However, the 4½ pulse overlaps (unfavorably)

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2b3u

D0(Ag)

S2(B2u)

6ag
1b1g
j b2u

i b2u
2b3u
6ag
1b1g
j b2u

Figure 6. Main conﬁgurations of (a) D0 and (c) S2 andrelevant conﬁgurations for one-photon ionization of D0 ←S2, (b) and (d).

Figure 5. (a) Temporal proﬁle of total photoelectron signal
in 3½­4½ TRPEI of pyrazine-h4. The observed proﬁle iswell explained by three components: single-exponential
decay of S2 (red); corresponding increase in S1 (blue) for
positive time delays; and single-exponential decay of S3(green) for negative time delays. The ﬁtting result is shown
as a solid line. (b) Time-evolution of photoelectron kinetic
energy (PKE) distribution, ·(t,E). (c) Time evolution of thephotoelectron angular anisotropy parameter ¢2(t,E).

with the S3­S0 band, and the 4½­3½ pulse sequence in thenegative time delays also creates a photoelectron signal.
Figure 5a shows the total photoelectron signal intensity as
a function of the pump­probe time delay. For positive timedelays with the 3½­4½ pulse sequence, the total signal intensity rapidly decays within the ﬁrst 30 fs and exhibits a plateau
at later times: our previous experiment in 1999 has shown that
this long-lived component has a ﬁnite lifetime of 22 ps forpyrazine-h4.27,53 For negative time delays with the 4½­3½
pulse sequence, the signal decays within 50 fs. These time
proﬁles, therefore, consist of three components corresponding
to the decay of optically excited S2 (red), the corresponding
growth of S1 (blue) populated by internal conversion from S2,
and the decay of S3 (green), respectively. Using least-squares
ﬁtting, the S2 ¼ S1 internal conversion time constants are
estimated as 23 « 4 fs for pyrazine-h4 and 20 « 2 fs forpyrazine-d4. The plateau region exhibits oscillatory featuresdue to vibrational quantum beats of Q6a in S1 (583 cm¹1).
Figure 5b shows the photoelectron kinetic energy distributions measured at each time delay. Although the S2 ¼ S1
internal conversion occurs within 30 fs, no marked change isobserved here. This is because photoionization predominantly

occurs as D0(n¹1) ← S1(n,π*) and D1(π¹1) ← S2(π,π*) and theenergy diﬀerences are almost identical between D1 and D0(0.88 eV)54 and between S2 and S1 (0.86 eV).55
Figure 5c shows the time and energy dependence of theanisotropy parameter ¢2(t,E); the positive (blue-green) andnegative (red) values correspond to preferential ejection of anelectron parallel and perpendicular to the probe laser polarization (eq 1). The energy-dependence of ¢2, a stripe of colors
at each time delay in Figure 5c, is the ﬁngerprint of theelectronic character. In the region of t < 30 fs, high-energy
electrons are ejected perpendicularly to the probe laser polarization (red), while low-energy electrons are distributed almost
isotropically (green). After 30 fs, the high-energy electron
distribution becomes isotropic, while low-energy electrons are
preferentially ejected parallel to the probe laser polarization(blue). The sudden change of the color at ca. 30 fs indicates theoccurrence of S2 ¼ S1 internal conversion, in agreement withthe analysis of the total electron signal.
In Figure 5c, the photoelectron kinetic energy component of
0.8­1.0 eV appearing within 30 fs is attributed to photoionization from S2(π,π*) to D0(n¹1). As shown in Figure 6, themain electron conﬁgurations of S2(π,π*) to D0(n¹1) cannot be
connected by a one-electron process. The occurrence of theD0(n¹1) ← S2(π,π*) ionization suggests that there are otherelectron conﬁgurations involved in the cationic and/or neutral
state. Within the Hartree­Fock approximation, the electronic
conﬁguration of pyrazine in its ground state (S0) is given by
1b21u 1a2g 1b22u 2a2g 2b21u 1b23g 3a2g 3b21u 2b22u 4a2g 2b23g 5a2g 3b22u 4b21u 4b22u 3b23g
 1b23u ð³Þ5b21u ðnÞ1b22g ð³Þ1b21g ð³Þ6a2g ðnÞ2b03u ð³ Þ1a0u ð³ Þ2b02g ð³ Þ

ð2Þ
where nonbonding and π orbitals are respectively denoted byn and π and all other orbitals are σ orbitals. Denoting the diﬀerence from eq 2, the leading conﬁgurations of D0 and S2 are1
þ1
expressed by 6a1
g and 1b1g 2b3u , respectively. To enable D0 ←S2 ionization, D0 and/or S2 must involve minor electron conﬁgurations with a singly occupied b2u orbital and two-electron
1
þ1
excitation from the leading conﬁguration: jb1
2u 1b1g 2b3u ( j =1 þ1
1, 2, 3, and 4) and 6ag jb2u ðj  5Þ are the candidates forD0 and S2, respectively as shown in Figures 6b and 6d. Inorder to ascertain this speculation, we performed ﬁrst-order
conﬁguration interaction and continuum multiple scattering

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Figure 7. Photoelectron kinetic energy distribution observed using the 3½ and 5½ pulses. Positive time delays
correspond to the case where the 3½ pulse precedes the
5½ pulse.

Xα (CMSXα) calculations, which indicated that the spectral
intensity of D0 ← S2 is 47% from 3b2u, 27% from 4b2u, and
26% from all virtual b2u ( jb2u, j ² 5).56
The characteristic negative ¢2 associated with the D0 ← S2transition assisted identiﬁcation of the S2 ¼ S1 internal conversion in pyrazine. Using CMSXα calculations, we found that¢ parameter diminishes with the electron kinetic energy owing
to the energy-dependent Coulomb phases but ¢ does not
become negative without a shape resonance, caused by a shortlived bound electronic state buried in the ionization continuum.
Our calculations indicate that there is a shape resonance at
3.5­4.0 eV above the ionization energy.56 Aromatic molecules
often have shape resonances within the range of excess energy
less than 10 eV, which should be taken into account in theinterpretation of TRPEI.
Figure 5b indicates that the Franck­Condon distribution
upon photoionization extends toward an unobservable negative
kinetic energy region. A probe photon with higher energy will
enable observation of the entire Franck­Condon envelope.
Thus, we revisited the same reaction using the ﬁlamentation
VUV light source. TRPES of pyrazine using the 3½ pump and
5½ probe is shown in Figure 7. In the positive time delay
region, photoionization from the S2(π,π*) and S1(n,π*) areclearly seen in the range of 1.5­2.5 eV. Notice that the S1component has shifted to higher photoelectron kinetic energies
in Figure 7 in comparison with Figure 5b. Since 5½ is strongly
absorbed by pyrazine, the 5½­3½ pulse sequence also produces
a signal in the negative time delay region. The three peaksin the energy range of 1.5­2.7 eV are of the 3s, 3pz, and 3py
Rydberg states.54,57 The distribution in the 0­2 eV range is dueto photodissociation from higher valence states excited with
5½, so that the photoelectron kinetic energy diminishes rapidly
as the vibrational wave packet moves away from the Franck­
Condon region.
2.3 S2 ¼ S1 Internal Conversion via Conical Intersection
in Benzene and Toluene. Benzene is a benchmark aromatic
molecule for theoretical and experimental studies of organic
compounds. The S2(1B1u) states of benzene and its derivatives
are short-lived owing to ultrafast S2 ¼ S1 internal conversion.

Figure 8. The time proﬁles of the photoionization signal
intensity for (a) benzene and (b) toluene. Photoelectron
signals are indicated by green dots with error bars. Time­energy maps of the photoelectron intensity, σ(t,E), for(c) benzene and (d) toluene. Time­energy maps of thephotoelectron angular anisotropies, ¢2(t,E), for (e) benzene
and (f ) toluene and ¢4(t,E) for (g) benzene and (h) toluene.
Data points for ¢2 and ¢4 with standard deviations smaller
than 0.2 are shown (see text). Reproduced with permission
from Ref. 58.

Let us examine the dynamics in comparison with the internalconversion in pyrazine.
Figures 8a and 8b show the 4½ pump and 3½ probe TRPES
of benzene and toluene, respectively.58 The broken lines are
single exponential decay functions convoluted with the crosscorrelation of the pump and probe pulses. The discrepancy
between the data points and the simulated curves indicate thatthe single exponential decay model does not adequately reproduce the observed time proﬁles in either case. The observed
decay proﬁles exhibit delayed responses, which correspond to
propagation of a wave packet from the Franck­Condon region
to the conical intersection region. The arrival times of the wave
packets at the seam of crossings are estimated as 33 fs forbenzene and 41 fs for toluene: the subsequent population decay
time is 32 fs for benzene and 43 fs for toluene.

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30 fs. Similar time dependence of ¢2 is also seen in Figure 8f
for toluene, for example at around 1.0 eV. The evolution of¢2(t,E) indicates that the electronic character gradually changes
along the out-of-plane distortion. Notice that the out-of-planedistortion eliminates the planar symmetry and, consequently,
distinction between the σ and π orbitals. Thus, diﬀerent electronic states at planar geometry are mixed at nonplanar geometries: the mixing is most likely with 1E1u and 1E2g.

Figure 9. Comparison of conical intersections in pyrazine
and benzene. The vertical arrows indicate photoexcitation,and the horizontal axes are the reaction coordinates. The
red and green correspond respectively to the diabatic S2and S1 states, and blue indicates contribution of other zeroorder electronic states.

Figures 8c and 8d show the time evolution of photoelectron
kinetic energy distributions extracted from a series of images
taken at diﬀerent time delays for benzene and toluene, respectively. The distributions consist of the S2 and S1 componentsthat are largely diﬀerent each other. The S2 component mainly
appears in the 0.5­1.5 eV range, while S1 < 0.3 eV. Ionizationfrom S2 and S1 occurs to the same cationic states, in which thevibrational energies are nearly conserved owing to the Franck­
Condon principle upon ionization and similar potential energy
surfaces between the neutrals and ions. Since internal conversion transforms the S2­S1 electronic energy diﬀerence into
the vibrational energy in S1, photoionization from S1 occurs tohighly vibrationally excited states of the cation leaving small
photoelectron kinetic energies.59,60 For both benzene andtoluene, close examination of the high-energy (1­1.5 eV) regionof the S2 component reveals oscillatory vibrational wave packet
dynamics: for example, in the case of benzene, the maximumof the energy distribution appears at 1.4 eV at 0 fs, 1.0 eV at
20 fs, and 1.3 eV at 30 fs.
Similarly to the case of pyrazine, the ultrashort S2 lifetime
implies that photoexcited benzene easily accesses an S2/S1
conical intersection region. Theoretical calculations predictedthat the minimum-energy S2/S1 conical intersection point(a prefulvenic form) is close, in energy and structure, to theS2 potential minimum.6,61,62 The minimum, however, is at a
nonplanar structure (a boat form) that diﬀers from the planar
structure of benzene (D6h) in S0.6,61,62 Consequently, a photoexcited benzene molecule rapidly deforms from a planar structure in the Franck­Condon region toward a nonplanar structure
along the steepest descent of the S2 potential energy surface
and undergoes a nonadiabatic transition in the vicinity of the
S2/S1 seam of crossings. This is a diﬀerent feature from thepyrazine case in which the Franck­Condon region is close to
the minimum energy conical intersection point at the planar
geometry (Figure 9). The hot S1 benzene produced by S2 ¼ S1
internal conversion is further funneled down to S0 via the S1/S0
conical intersection in <10 ps.59,62,63
Close examination of Figure 8e for benzene reveals that ¢2varies with time, most clearly around 0.7 eV; ¢2 is negative at
t = 0 and gradually increases with time to be positive around

3. Restoration of Sharp Photoelectron Angular Distribution
from Broad Angular Distribution Observedfor Rotating Molecules
Chemists consider reaction mechanisms having in mind
the electron orbitals of a molecule ﬁxed in space; however,real molecules are randomly oriented in gases and liquids andthis leads to inevitable directional averaging in physicalmeasurements. It is an experimental challenge to overcome
this diﬃculty and “watch” electronic structures and dynamics inthe molecular frame. In one-photon excitation of randomly
oriented molecules with a linearly polarized light, photoexcitedmolecules are weakly aligned in space with their transition
dipole moments distributed by cos2 º around the polarizationof light, where º is the angle of the transition dipole andpolarization. The 4th order Legendre term in eq 1 arises fromthe alignment eﬀect. However, the alignment created by onephoton excitation is so weak that, even if a photoelectron
angular distribution has sharp structures in the molecular frame,
the observed distribution in the laboratory frame is considerably
blurred by the broad molecular axis distribution. Althougheven the blurred distributions provide valuable insights into
nonadiabatic dynamics as discussed in the previous section,
I discuss in this section restoration of a sharp photoelectron
angular distribution, seen by an observer on the rotating
molecule, from the blurred distributions. The key element forsuccessful restoration is accurate information about a pointspread function, which projects a single point in the original
sharp image into a ﬁnite region in a transformed (blurred)
image. In the present case, the point-spread function is givenby the axis alignment factor A(t). The molecular axis distribution P(») in the laboratory frame is expressed as P(t,») =·/4³[1 + A(t)P2(cos »)] using A(t). The important advantage
of one-photon excitation is that A(t) can be rigorously calculated
and reliable analysis can be performed.
In quantum mechanics, localization of a particle in a freespace is realized by superposition of multiple eigenfunctions
with diﬀerent momenta; however, localization is nonstationary
and the distribution function is rapidly broadened owing tophase dispersion of diﬀerent momentum components. Likewise, sharp directional alignment or orientation of a molecule isassociated with a rotational wave function with a broad angularmomentum distribution. In one-photon excitation of molecules,
the selection rule of ¦J = 0, «1 creates a superposition of only
a few rotational states and a broad molecular axis distribution(J is the rotational angular momentum quantum number). The
quantum mechanical interference of rotational wave functions
creates peculiar time dependence of the molecular axis alignment factor, A(t). Let us examine it for the S1(n,π*) ¼ T1(n,π*)intersystem crossing in pyrazine. This process occurs in 110 ps,
which is comparable to the rotational period, 82 ps, of this

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These measurements provided suﬃcient number of experimental observables to determine 11 values of the amplitudes andphases. We excited NO(2¬1/2) to the A(2­+) state with a 226 nm
pulse, and probed by one-photon ionization at various probe
wavelengths from 323 to 242.5 nm to change the photoelectron
kinetic energy from 0.05 to 1.33 eV.67,68 One technologicalchallenge was that the ¢2, and ¢4 parameters needed to be
determined with accuracies of 0.01. In order to achieve thisaccuracy, we developed the 1 kHz CMOS camera and centroiding calculations using ﬁeld programmable gate arrays andintegrated each image over 106 laser shots.31
The molecular frame photoelectron angular distributions
thus determined are shown in Figure 12 for two diﬀerent probe
photon polarization directions and photoelectron kinetic energies.67,68 Photoionization with the polarization parallel to thebond axis induces photoemission on the oxygen side at low
energy, while photoemission to the nitrogen side increases withenergy. Polarization perpendicular to the bond axis creates abroad photoelectron angular distribution near the threshold,while it sharpens distributions along the probe laser polarization at higher kinetic energies.
4. Time-Resolved Photoelectron Spectroscopy of Liquids

Figure 10. (a) Two-dimensional slice image of photoelectrons in the [1 + 2¤] PEI of pyrazine via the S1 B3u(n,π*) 0°
level observed at a time delay of 30 ps. The original image
was integrated for 80000 laser shots. (b) Time evolution of
the three major components in the [1 + 2¤] PEI. Circles ( ),triangles ( ), and squares ( ) represent the angle-integrated
intensity for the outer (PKE = 643 meV), middle (101meV), and inner rings (37 meV), respectively. Solidlines are a simulation taking into account the rotational
coherence. (c) ¢20/¢00 as a function of 5 deﬁned in the text.
Reproduced with permission from Ref. 64.

molecule.28,64,65 After exciting pyrazine to the zero vibrational
level in S1, we observed two-photon ionization from the S1 andT1 states using the 400 nm probe pulse, in which ionization wasenhanced by accidental resonance with the Rydberg states.64
The photoelectron signal intensities of S1 and T1 in Figure 10b
exhibit respectively a gradual decay and growth as a functionof time, in which characteristic spikes due to A(t) appeared(The S1 state is probed via both 3s and 3p Rydberg states,54,57
which exhibit opposite sign of the rotational coherence spikes).
Similar signatures of A(t) is also seen in the photoelectron
angular anisotropies (represented by ¢20/¢00 in this case) in
Figure 10c.
We demonstrated restoration of molecular frame photoelectron angular distribution using [1 + 1¤] ionization of NO via theA(2­+) state as an example. A free electron wave in Coulombic
interactions with a diatomic cation is expressed using theelectron partial waves such as sσ, pσ, pπ, and others as thestandard method. We derived the formulae that relate ·(t,E),¢2(t,E), and ¢4(t,E) with A(t) and the amplitudes and phasesof partial waves,66­68 and we measured these observables forvarious time-delays and probe photon energies (Figure 11).

4.1 TRPES at Ultralow Kinetic Energies. It is highlyinteresting to explore solution chemistry using TRPES. On theother hand, a crucial quantity in photoelectron spectroscopy ofcondensed matter is the escape depth of an electron. Although
excitation light has a large penetration depth into the bulk
material, electrons created far from the surface are unlikely toescape into vacuum owing to inelastic scattering in the bulk.
The inelastic scattering cross section takes the maximum value
at around 50­100 eV and diminishes in both lower and higher
energy sides.69­74 For example, Figure 13 shows the integral
cross sections of the inelastic scattering of an electron, withthe electron energy loss larger than 1 eV, in amorphous ice.75
The result indicates that the cross section diminishes by nearly
three orders of magnitudes for E < 8 eV. Thus, one may expectthat the electron escape depth in liquid water increases at low
electron kinetic energy. However, an electron undergoes extensive elastic scattering in liquid water at low kinetic energies,
and the actual escape depth has not been determined yet.
Extensive studies on the escape depth of an electron fromliquid water is in progress in our laboratory; a reasonableestimate is 2­10 nm in the kinetic energy region less than 10 eV.
This depth may seem rather short; however, the high relative
permittivity of water eﬀectively screens electrostatic interactions by hydration, making the liquid properties rapidly
approach those of the bulk at small depths from the surface.
4.2 Charge Transfer to Solvent Reaction from I¹ to Bulk
Water. In order to characterize TRPES of liquids, we studied
the charge transfer to solvent (CTTS) reaction from I¹ to bulk
water, which has previously been studied using conventionaltransient absorption spectroscopy.76,77 When I¹(aq) is photoexcited to a metastable excited state below the conduction
band, an excess electron cannot be ejected freely into the bulk;
the electron cloud can, however, penetrate into bulk water tobe trapped as a hydrated electron. Internal conversion to theground state of I¹(aq) competes with this CTTS reaction. Whenaccommodating the excess electron, a local hydrogen-bonding

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Bull. Chem. Soc. Jpn. Vol. 87, No. 3 (2014)349

Figure 11. (a) The excitation scheme from NO (X2¬1/2, v″ = 0) to NO+ (1­+, v+ = 0) via the A (2­+, v¤ = 0) state. Panels (b), (c),and (d) show axis alignment factor A(t) and time energy maps of photoelectron intensity and anisotropy parameters measured asfunctions of photoelectron kinetic energy and pump­probe time delay. ·(t,E) in (b) represents the normalized values at each PKE.
The anisotropy parameters were measured at around 4.2 and 8.4 ps with a time window of 0.6 ps, and the anisotropy parameters atother time delays were calculated from these experimental values to illustrate their time dependences. Reproduced with permission
from Ref. 67.

Figure 12. Molecular frame photoelectron angular distributions reconstructed from time­energy mapping of photoelectron
anisotropy parameters for the σ (a) and π (b) channels. Reproduced with permission from Ref. 68.

network has to change drastically by reorienting water molecules. For the studies of CTTS dynamics strongly coupled withsolvation dynamics, I¹(aq) is a suitable solute, as it has nointernal degrees of freedom besides electronic motion. We

excited I¹(aq) in water with a pump pulse (243­226 nm) andphotodetached negatively charged transients with a timedelayed 260 nm probe pulse (Figure 14).19 Figure 15 shows
the photoelectron spectrum measured as a function of the timeBull. Chem. Soc. Jpn. Vol. 87, No. 3 (2014)

AWARD ACCOUNTS

Figure 13. Integral cross section (shown by dots) ascribedto the sum of dissociative attachment (DA), vibrational
excitation above 1 eV energy loss, electronic excitations(electr), as well as ionization (ion) processes in amorphous
ice. Adopted from Ref. 75.

Figure 15. 3D plot of the photoelectron kinetic energy distribution as a function of time in [1 + 1¤] TRPES of 0.14 M
aqueous NaI solution. The pump and probe wavelengths
are 243 and 260 nm, respectively. Reproduced withpermission from Ref. 19.
photoelectron signal (arb. unit)

350

1

1

H2O

D2O

H2O

0.5

0

0

2

D2O
0
0

50

100

delay time/ps

Figure 14. Schematic energy diagram for time-resolved
photoelectron spectroscopy of a charge transfer to solvent(CTTS) reaction. Reproduced with permission from
Ref. 78.

delay. The energy spectrum is initially broad, but the width
diminishes rapidly and the intensity decreases gradually. Ifelectron inelastic scattering in water is extensive, the photoelectron kinetic energy distribution should exhibit a decelerated
component irrespective of the pump­probe time delay. However, the result exhibits no signature of decelerated electrons,
indicating that the inﬂuence of inelastic scattering on theenergy distribution can be neglected.
Figure 16 shows the total electron intensity observed as afunction of delay time for the CTTS reactions in H2O andD2O.78 The decay proﬁles clearly indicate that there are atleast three elementary steps involved in the CTTS reaction,and the time constants are diﬀerent between the two solvents.
The time-dependences of the photoelectron signal intensity andspectral shifts are explained by a model assuming two reaction
intermediates of a solvent separated and contact pair states.78
The observed decay is ascribed to geminate recombination of

Figure 16. Photoelectron signal intensity as a function ofpump­probe time delay observed for the charge transfer tosolvent (CTTS) reaction of I¹ to bulk water: the sample
solutions were 0.1 M aqueous NaI solution in H2O (black)and D2O (blue). The schematic energy diagram of theexperiment is shown in Figure 14. The cross correlation of
the pump and probe laser pulses was ca. 300 fs. A hemispherical electron energy analyzer was used to measurea photoelectron spectrum at each time delay, and thespectrum was integrated over photoelectron kinetic energy
to obtain the each data point. Reproduced with permission
from Ref. 78.

an electron with a neutral iodine atom from the CTTS, solvent
separated, and contact pair state. The time constants extracted
from TRPES (Figure 17) are in good agreement with thosefrom transient absorption spectroscopy.76,77 Thus, although the
probing depth of TRPES is not established yet, it is strongly
suggested that TRPES at ultralow kinetic energy (<5 eV)probes bulk solution chemistry.
4.3 Electron-Binding Energies of Solvated Electrons.
The CTTS reaction in bulk water discussed in the preceding
section ultimately creates hydrated electrons.19,79­98 The hydrated electron is the most important transient species inradiation chemistry and biology; its electron-binding energy,
however, has not been measured by any methods. In the 1990s,

T. Suzuki

Figure 17. Graphical presentation of our kinetic modelwith some representative time constants for the chargetransfer to solvent (CTTS) reaction of I¹ to bulk water. The
observed experimental results can be explained equallywell by a kinetic model presented here and a diﬀusion
model. The latter involves solving a diﬀusion equation
with an assumed interaction potential, e.g., a Morse potential. Reproduced with permission from Ref. 78.

researchers found that the electron-binding energy of negatively charged water clusters varies approximately as n1/3
where n is the number of water molecules in the cluster. This
trend agrees qualitatively with a model of an electron in a
spherical dielectric cavity. In the 1990s, the electron-binding
energy of a hydrated electron in bulk water was estimated to be
3.3 eV by extrapolating the vertical electron-binding energies
of negatively charged water clusters to the inﬁnitely large sizeof the cluster.87 Theoretical calculations, however, have refuted
this estimate by showing that the electron-binding energy
varies with n1/3 even when the excess electron is trapped at
the surfaces of clusters.90,99,100 Since the cluster anions are
generated by electron attachment to cold neutral clusters inmolecular beams, the energetic penalty is large for an excess
electron to penetrate into a cluster by disrupting the hydrogenbonding network; consequently, an electron is trapped at thesurface of cold clusters. Later studies have shown that there
are three diﬀerent isomers (I­III) for negatively charged water
clusters. The isomers I­III have three diﬀerent asymptotic
values of electron-binding energies at the inﬁnitely large sizesof the clusters, in which isomer I is regarded as a precursor of ahydrated electron in the bulk,101 while isomer II seems related
to a hypothesized surface state of a hydrated electron.102,103
A more recent study has revealed that the isomer I has two
diﬀerent species known as isomer Ia and Ib; the extrapolationof the electron-binding energy to the inﬁnite cluster size of Ib isin the region of 3.5­4.0 eV (Figure 18).94,104
Our ﬁrst TRPES experiment of aqueous NaI solution
estimated the vertical electron-binding energy of a hydrated
electron in bulk water to be 3.3 eV (Figure 15),19 in excellent
agreement with the estimate made from the cluster values.86,87
However, a low signal-to-noise ratio and electrokinetic charging of a liquid beam limited the accuracy. In order to achieve
higher precision and reliability for liquid TRPES, we haveconstructed a magnetic bottle time-of-ﬂight (TOF) spectrometer

Bull. Chem. Soc. Jpn. Vol. 87, No. 3 (2014)351

Figure 18. Vertical binding energy of a hydrated electron inbulk water observed in this work compared with the cluster
values by Ma et al.104 The original extrapolation by Coe
et al. is also indicated by a broken line.87 Reproduced withpermission from Ref. 19.

Figure 19. Photoelectron spectra of solvated electrons inbulk solutions: H2O (black), D2O (blue), methanol (green),
and ethanol (red). Open circles are experimental data andsolid lines are the best-ﬁt Gaussian functions obtainedby least-squares ﬁtting. There was no signal in the energy
region of 0­1.5 eV. Reproduced with permission from
Ref. 105.

(ﬂight length: 1 m), which collects 50% of photoelectrons
emitted from the liquid beam and enables measurement of theentire photoelectron energy spectrum on a shot-to-shot basis.105
In combination with a 100 kHz DUV femtosecond laser weconstructed, the new spectrometer improved the signal countlevel by four to ﬁve orders of magnitude in comparison with
our old system. The spectra of solvated electrons in H2O, D2O,
methanol, and ethanol measured at a pump­probe time delayof 2 ns are shown in Figure 19. A striking feature of these
spectra is their symmetric Gaussian shapes: the photoabsorption spectra of solvated electrons in bulk solutions as well as
photoelectron spectra of water cluster anions exhibit asym-

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AWARD ACCOUNTS

Table 1. Vertical Binding Energies (eV) of SolvatedElectrons
Solvent
H2O
D2O
Methanol
EthanolLiquid
3.4
3.5
3.4
3.3

Estimate from cluster anion
3.3 (Ia)a), 4.0 (Ib)b)
®
2.6c)
®

a) Ref. 87. b) Ref. 104. c) Ref. 106.

metric band shapes, which are ﬁt by a Gaussian and Lorentzian.
The electron-binding energies of solvated electrons determinedfrom these high-precision measurements are listed in Table 1.
As seen in Table 1, the electron-binding energy of a solvatedelectron in methanol is 3.4 eV, which is clearly larger than thevalue, 2.6 eV, estimated from the methanol cluster data.106
The result indicates that the excellent agreement between
the electron-binding energy of a hydrated electron in bulk
solution and the estimate from water cluster anions cannot be
generalized to other solvents. Molecular cluster anions produced in supersonic molecular beams are at ultralow temperatures, and their thermodynamic properties are generally diﬀerent from bulk solution at an ambient temperature. Therefore, it
is possible that the agreement observed for water is fortuitous.
5. Summary
Time-resolved photoelectron imaging (TRPEI) enables
eﬃcient and accurate measurements of photoelectron energy
and angular distributions as a function of the pump­probe timedelay. Time­energy mapping of photoelectron angular anisotropy clearly reveals rapid changes of electron conﬁguration
during the course of photoinduced dynamics. The S2 stateof pyrazine exhibited immediate population decay from theFranck­Condon region, while the S2 state of benzene andtoluene decayed after ﬁnite induction time due to wave packet
motions from the Franck­Condon region to conical intersection
region. The results illustrate the importance of the locationsof the conical intersections on the potential energy surfaces
in internal conversion dynamics. It has been shown that thephotoelectron angular anisotropy is inﬂuenced by shape resonances with metastable bound states in the continuum.
Time-resolved photoelectron spectroscopy (TRPES) of
liquids provides new opportunities for ultrafast spectroscopic
studies of electron dynamics under wet conditions. TRPES ofthe charge transfer to solvent reaction from I¹ to bulk water
well demonstrated the usefulness of this methodology for thestudies of solution chemistry. The electron-binding energies
of solvated electrons in four polar protic solvents were ﬁrmly
established by liquid TRPES. The method is expected to beparticularly useful for elucidating electron-transfer processes
and redox reactions. Similarly with TRPES in the gas phase,the time­energy mapping of photoelectron angular anisotropy
becomes an invaluable tool in studying electronic dynamics
in solution, and its technical development is underway in ourlaboratory.
I would like to thank my present and past group members
who made great contribution to TRPES in our laboratory, Y.
Suzuki, T. Fuji, Y. Tang, T. Horio, H. Kohguchi, Y. Ogi, S.-Y.

Liu, H. Shen, K. Sekiguchi, and N. Kurahashi. I also thankProfessors V. Bonacic-Koutechy and R. Mitiric for collaboration on molecular dynamics on the ﬂy.
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Toshinori Suzuki obtained his Ph.D. degree in 1988 from Tohoku University. He then became a
research associate at the Institute for Molecular Science (IMS), between 1988 and 1990, and a
JSPS fellow for research abroad to carry out research on molecular beam scattering at Cornell
University and the University of California, Berkeley, between 1990 and 1992. He returned toJapan as an associate professor at IMS, where he started his independent research group on
chemical reaction dynamics in 1992. In 2001, he moved to RIKEN to be a chief scientist andextended his research further. In 2009, he became a professor of physical chemistry at Kyoto
University to educate the next leaders of Japanese science, and from 2012, he is serving as a Chairof the Division of Chemistry, Graduate School of Science. He received Broida Award, IBM
Science Award, JSPS Award, Commendation for Science and Technology by MEXT.