No. 1]

Proc. Jpn. Acad., Ser. B 89 (2013)

1

Review
Visualization of chemical reaction dynamics:
Toward understanding complex polyatomic reactions
By Toshinori SUZUKI*1,*2,†(Communicated by Hiroo INOKUCHI, M.J.A.)

Abstract: Polyatomic molecules have several electronic states that have similar energies.
Consequently, their chemical dynamics often involve nonadiabatic transitions between multiple
potential energy surfaces. Elucidating the complex reactions of polyatomic molecules is one of themost important tasks of theoretical and experimental studies of chemical dynamics. This paper
describes our recent experimental studies of the multidimensional multisurface dynamics ofpolyatomic molecules based on two-dimensional ion/electron imaging. It also discusses ultrafast
photoelectron spectroscopy of liquids for elucidating nonadiabatic electronic dynamics in aqueous
solutions.
Keywords: molecular beam, chemical reaction, nonadiabatic transition, imaging, scattering,
photoelectron spectroscopy

1. Introduction: imaging reactive
molecular scattering

Liquid is the most important phase of matter inchemistry and biology. However, it is very diﬃcult to
study bimolecular reaction dynamics in liquids dueto the billions of collisions occurring per second inliquids. These collisions obscure the initial conditionsof the reacting pair and obliterate the nascentproduct state distributions. Consequently, the mostdetailed studies of bimolecular reactions employ
crossed atomic and molecular beams in vacuum.
The crossed-beam technique prepares well-deﬁned
quantum states and velocities of reactants andinterrogates the products and their velocities. Thismethod was ﬁrst demonstrated in 1955.1) It was
initially applied only to the reactions of alkali metal
atoms due to its low detection sensitivity of products;
this period of research is often referred to as the“alkali age” of molecular beam research. In thelate 1960s, a universal detector was developed. Thisdetector employed electron bombardment ionization*1
Department of Chemistry, Graduate School of Science,
Kyoto University, Kyoto, Japan.
*2
RIKEN, Saitama, Japan.
†
Correspondence should be addressed: T. Suzuki, Department of Chemistry, Graduate School of Science, Kyoto University,
Kyoto 606-8502, Japan (e-mail: suzuki@kuchem.kyoto-u.ac.jp).

doi: 10.2183/pjab.89.1©2013 The Japan Academy

and quadrupole mass analysis,2) and it enabled thevelocity distribution (i.e., speed and angular distributions) of virtually any chemical species to bemeasured with mass selection. Crossed-beam experiments using this universal detector revealed fundamental aspects of chemical reaction dynamics forwhich the Nobel Prize for Chemistry was awarded
to Lee and Herschbach (with Polanyi) in 1986.
However, the universal detector is unable to selectthe quantum (rotational, vibrational and electronic)
states of reaction products, which limits the usefulness of this methodology. Since the 1970s, eﬀorts
have been made to develop new experimental
techniques to perform quantum-state-selective measurements of the velocity distributions of reaction
products.3)–8) Of these techniques, two-dimensional
ion imaging is the most powerful.9),10)
Experimental and theoretical studies since the1950s have greatly advanced our understanding of
atom D diatom reactions such as H D H2 andF D H2.11)–18) Extension of such studies to atom Dpolyatom and diatom D diatom systems is critical for
elucidating the multidimensional dynamics of polyatomic molecules. When studying these reactions, it
is essential to measure the velocity distributions ofquantum-state-selected reaction products. To understand why, we compare the atom D diatom reaction,
O(1D2) D H2 ! OH D H and the atom D polyatom

2

T. SUZUKI[Vol. 89,

Fig. 1. Center-of-mass speed and angular distribution of OH produced by O(1D) D CH4 reaction at a collision energy of 6.5 kcal/mol.
The distribution was determined by simulation of the experimental data obtained by the universal crossed-beam apparatus.
Reproduced with permission from Ref. 23.

reaction, O(1D2) D CH4 ! OH D CH3. If the O(1D2)and H2 beams collide with a well-deﬁned kinetic
energy, the velocity distribution of H can be used
to determine the internal energy distribution of OH
by applying the energy and momentum conservation
laws.19)–22) However, for the reaction O(1D2) D CH4,
mass-selected velocity measurements of OH or CH3
do not provide the internal energies of the products
OH or CH3 because their velocities are determinedby the total internal energies of both OH and CH3.
Figure 1 presents the velocity distribution of
the OH product of the reaction O(1D2) D CH4 !
OH D CH3 measured using the universal detector.23)
The anisotropic angular distribution indicates thatthe reaction does not generate any long-lived reaction
intermediate. On the other hand, this structurelessspeed distribution is not very useful for elucidating
the dynamics. The following explanation describes
how quantum-state-selected measurements of products provide deeper insights into the dynamics of thereaction O(1D2) D CH4.
The O(1D2) state is a highly reactive metastablestate of an O atom. Since O(1D2) is the primary
photodissociation product of stratospheric ozone,
O3 D h8 ! O(1D2) D O2(1"g), the O(1D2) reaction isof great interest for atmospheric chemistry. O(1D2)
reactions with an aliphatic hydrocarbon are representative “insertion” reactions as they proceed via acomplex produced by inserting O(1D2) into a chemical bond. The high reactivity of O(1D2) originatesfrom the attractive long-range force in the insertion
pathway: in the case of O(1D2) D CH4, O(1D2) can
attack the C–H bond of methane to form a methanolintermediate that subsequently breaks up into
CH3 D OH (the insertion pathway). On the other
hand, product analyses performed in the 1960s and1970s suggest the existence of a direct “abstraction”
pathway:24),25) O(1D2) attacks the H atom collinearlywith the C–H bond and abstracts the H atom toform CH3 D OH without any reaction intermediate(the abstraction pathway). The abstraction pathway
has an activation barrier. The origins of thesetwo pathways can be understood by considering
the reaction O(1D2) D H2(1’gD) ! OH(2&) D H(2S).
The electronic symmetry of this system is either 1AB
or 1ABB depending on the reﬂection symmetry of theelectronic wave function of O(1D2) relative to themolecular plane deﬁned by the three atoms because
the electronic wave function of H2(1’gD) is symmetricfor reﬂection. Since O(1D2) is a ﬁve-fold degeneratestate, ﬁve potential energy surfaces are created byinteraction with H2. Of these, the 11AB state iscorrelated with the ground electronic state of H2O
and provides the insertion pathway, while the 11ABB
state leads to the abstraction pathway. A similar
symmetry argument applies to the local symmetry

No. 1]Visualization of chemical reaction dynamics: Toward understanding complex polyatomic reactionsof the O, C and H atoms in O(1D2) D CH4(1A1) !
OH(2&) D CH3(2A2BB) and it explains the duality of
the reaction pathway.
In the early 1990s, we used infrared diode laser
absorption spectroscopy to measure the vibrational
state distribution of CH3 radicals in the reaction
O(1D2) D CH4 ! OH D CH3.26) Radicals were observed in low vibrational states of the 82 (out-ofplane bending) mode, even though the structure of
the CH3 moiety changes from pyramidal to planar
during the reaction.26) This result contrasts with theinverted vibrational distribution of OH.27) Thesestudies of the rovibrational state distributions of the
products could not identify the elusive abstraction
pathway.26),27)
The scattering distributions (especially the
backward scattering distribution) of the products
are critical for identifying the abstraction pathway.
Since the insertion pathway has long-range attractive
interactions between O(1D2) and CH4 in the entrance
channel, collisions with large impact parameters (i.e.,
glancing collisions) should dominate the reaction.
Consequently, the CH3 products of the insertion
pathway are strongly scattered in the forward
direction (i.e., the initial beam direction of CH4). If
the CH3OH complex has a suﬃciently long lifetime,
it will rotate before breaking up into CH3 and OH,
so that the angular distribution of OH will extend
in the backward direction.28) On the other hand,the abstraction pathway has an extremely short
reaction time so that the product angular distribution is predominantly determined by the transition
state geometry. The transition state has a nearlycollinear O–H–C geometry, which provides backward scattering. The angular distribution in Fig. 1
exhibits strong forward scattering, indicating that
insertion is the main pathway and that the methanolcomplex has a very short lifetime. The distribution
reveals the backward scattering component, but it
is not clear whether the back-scattered products
originate from the “osculating” (short-lived) CH3OH
complex28) in the insertion pathway or from an
abstraction pathway.
In order to investigate O(1D2) reaction withmethane in greater detail, we constructed a crossed
molecular beam apparatus with two-dimensional
ion imaging, as schematically shown in Fig. 2.29)–32)
The key component of this apparatus is the intenseO(1D2) beam source that employs 157 nm photodissociation of O2 [O2 D h8 ! O(1D2) D O(3PJ)]. O2
is dissociated in the supersonic expansion of thecarrier (rare) gas to maximize the ﬂux of O(1D2) and

3

Fig. 2. Schematic illustration of crossed molecular beam ion
imaging experiment for studying the O(1D2) D CD4 ! OD D
CD3 reaction.

control their velocities; since the speed of O(1D2)essentially becomes the same as that of the carriergas, the collision energy between O(1D2) andmethane can be varied by changing the rare gas(He, Ne, Ar, or Kr) used as the carrier. The O(1D2)
atomic beam is crossed with a CD4 molecular beam ina scattering chamber33),34) and the CD3 product isstate-selectively ionized by resonance-enhanced multiphoton ionization via the 3pz Rydberg state using atunable nanosecond UV laser. (CD4 is used because
CD3 radicals are less predissociative than CH3 in the
3pz Rydberg state. The CD3D is also experimentallymuch less aﬀected than CH3D by background signals
from hydrocarbon contaminants from sources such as
pump oils.) The three-dimensional distribution of
CD3D ions is accelerated in an electric ﬁeld andprojected onto a two-dimensional position-sensitive
detector.33),34) The observed image is corrected forthe apparatus function and numerically inverted tothe speed and angular distributions of CD3.
Figure 3 shows examples of the speed andangular distributions inverted from the observed
images of the CD3 product in the vibrational groundstate.33) Figures 3(a)–(c) correspond to the ﬁnal
rotational states of J F 3, 5 and 7, respectively. All
these distributions exhibit strong forward scattering,in agreement with Fig. 1. The striking feature of the
distributions in Fig. 3 is that the backward scattering
component exhibits a discrete speed distribution. The
kinetic energy of the state-selected CD3 is related tothe internal energy of OD by the following energy
conservation relation:

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T. SUZUKI[Vol. 89,

Fig. 3. False-color map of scattering distribution of the CD3 product in the vibrational ground state. The rotational states of the
products are (A) J F 3, (B) 5, and (C) 7. The left and right sides of each image respectively show the observed image and the twodimensional slice of three-dimensional scattering distribution. The yellow arrows indicate the directions of the atomic and molecularbeams. Forward scattering means ejection of a CD3 radical in the direction of the CD4 beam. Reproduced with permission from
Ref. 33.

H þ Ecoll  Erovib ðCD3 Þ  Etrans ¼ Erovib ðODÞ ½1
where "H is the exothermicity of the reaction, Ecoll isthe collision energy, Erovib(CD3) is the rovibrationalenergy of CD3, Etrans is the total translational energyof both products, and Erovib(OD) is the rovibrationalenergy of OD. The observed speed distributions of
CD3 reveal that the counterpart OD radicals areformed in the vibrationally excited states of v F 4–6.
The strong vibrational excitation of OD is typicalfor an abstraction reaction with an early barrier.
The discrete ring structure indicates that rotationof OD is only weakly excited. This weak rotational
excitation is indicative of the collinear transition
state that is expected for the abstraction pathway;
the observed feature of backward scattering is rather
similar to that of F(2P) D CD4.35) The abstraction/
insertion ratio diminishes for higher rotational statesin Fig. 3 because the abstraction process has weakerrotational excitation than the insertion process.
The forward scattering is structureless, even in ourstate-resolved measurements. This implies that therotational excitation of the OD counterpart is quite
strong in the insertion mechanism, in agreement withlaser-induced ﬂuorescence measurements of OH forO(1D) D CH4.27)
The angular distributions of CD3 in the excited
vibrational states of 82 (out-of-plane bending) mode
(data not shown) exhibit essentially the samecharacteristics as the vibrational ground state.
However, as shown in Fig. 4, the CD3 products inthe excited state of a symmetric C–D stretchingvibration (v1) do not exhibit a discrete speed
distribution in the backward direction, indicatingthat CD3 (v1 F 1) is produced only in the insertion
pathway; the excitation of C–D stretching is ascribedto vibrational coupling in the methanol intermediate.

Fig. 4. Scattering distribution of the CD3 (v1 F 1) products. The
Q-branch peak of the 111 vibronic band was employed in thismeasurement and the oﬀ-resonance image was subtracted. The
left and right sides of each image respectively show the observed
image and a two-dimensional slice of the three-dimensional
scattering distribution. Comparison with Fig. 3 reveals that the
backward scattering component is missing. Reproduced withpermission from Ref. 33.

The strong forward scattering in Fig. 4 indicates that
the CD3OD complex has an extremely short lifetime.
The vibrational distribution of CD3 measured by thecrossed-beam experiment is consistent with weak
vibrational excitation of CH3 observed by infrared
diode laser spectroscopy.26) We measured the rotational distribution of CD3 produced by crossed-beam
scattering and found that CD3 radicals are createdin low K states, where K is the quantum number of
the angular momentum projected on the top axisof the CD3 radical. Low K states correspond to
tumbling rotation of CD3.33) Very recently, we
performed a similar crossed-beam imaging experiment on O(1D2) D CH4 ! OH D CH3 and observed

No. 1]Visualization of chemical reaction dynamics: Toward understanding complex polyatomic reactions

Fig. 5. Graphical presentation of the two reaction pathways.
Insertion occurs with a large impact parameter and results inforward scattering of rotationally excited products. Abstraction
reaction occurs via a collinear transition state and results in the
backward scattering of weakly excited products.

essentially the same characteristics as O(1D2) D
CD4 ! OD D CD3. Figure 5 shows a graphical depiction of the reaction mechanism of O(1D2) D
CH4 ! OH D CH3.
Theoretical estimates of the reaction barrierheight have ambiguities of ca. 2 kcal/mol. Ab initio
calculations using CASPT2 and the basis set 6311G(2df, 2pd) predict that the insertion pathway isbarrierless, whereas the abstraction pathway has abarrier height of 1.2 kcal/mol.36) Restricted internallycontracted multireference conﬁguration interaction
calculations at the complete basis set limit predictthat the insertion pathway will have a barrier heightof 1.47 kcal/mol (without correcting for the zeropoint energy).37) Further experimental studies are
currently being conducted in our laboratory toinvestigate the reaction barriers and the collisionenergy dependence of the dynamics. Accurate dynamical calculations of the ab initio potential energy
surfaces are highly computationally demanding, yetthey will be very useful for understanding the
dynamics.
2. Ultrafast photoelectron imaging: real-timeobservation of nonadiabatic dynamics

Although sophisticated crossed-beam ion imaging experiments provide ﬁne details of reaction
dynamics, they observe only the products after areaction occurs. Some important aspects of chemical
reaction dynamics can be studied only by real-time5

observations. One example is the nonadiabatic
dynamics of polyatomic molecules. The Born–
Oppenheimer approximation facilitates a quantummechanical description of a chemical reaction38) byenabling the equations of motions for electrons andnuclei to be separated. In the Born–Oppenheimer
approximation, chemical reactions are viewed asclassical trajectories or quantum-mechanical wave
packet motions of the nuclear geometry on electronic
potential energy surfaces. However, the Born–
Oppenheimer approximation breaks down whendiﬀerent electronic states are close in energy; thisgives rise to complex yet interesting nonadiabatic
dynamics. Nonadiabatic transitions frequently occur
in polyatomic molecules because they have several
electronic states with similar energies and theirmultidimensional potential energy surfaces frequently undergo avoided crossings. It is thus important to identify nonadiabatic transitions inexperimental and theoretical studies of polyatomic
reactions.
Real-time observations of bimolecular reactions
are not currently feasible because the timing ofcollisions cannot be controlled more accurately than
the reaction time. Consequently, bimolecular dynamics is studied only from the asymptotic states of
products using the crossed molecular beam scattering, as described in the previous section. On the other
hand, ultrafast spectroscopy permits real-time observation of the nonadiabatic dynamics of photoinduced unimolecular reactions.
Pyrazine (C4H4N2) is one of the most fundamental heteroaromatic molecules.39)–49) Its broad
S2(1B2u, ::*)–S0(1Ag) photoabsorption spectrum
suggests that the S2 state undergoes ultrafast
electronic deactivation (internal conversion) within
30 fs. This ultrafast deactivation originates from theconical intersection between the S2 and S1(1B3u, n:*)potential energy surfaces (see Fig. 6).41) In a diatomic
molecule, the von Neumann–Wigner noncrossing ruleprohibits degeneracy of electronic states with thesame symmetry so that their potential energy curves
have avoided crossings.50) For a nonlinear polyatomic
molecule with N vibrational degrees of freedom,
potential energy surfaces can be degenerate to form aseam of crossings in N–2 dimensional space (known
as a conical intersection). The conical intersection is
the most important topographical feature of multidimensional surfaces of polyatomic molecules.51)–55)
Ultrafast internal conversion of pyrazine via conicalintersection has been extensively investigated theoretically.39)–49)

6

T. SUZUKI[Vol. 89,

IðE; ; tÞ ¼

Fig. 6. Conical intersection of S2 and S1 adiabatic potential
energy surfaces of pyrazine in the two-dimensional spacespanned by Q10a and Q6a. Reproduced with permission from
Ref. 47.

Since a nonadiabatic transition abruptly alters
the electronic character, it is most sensitively
detected by measuring the electron distribution.
However, the total electron density of a molecule is
not the most suitable observable for this purpose
because the total electron density distribution ina molecule does not change at the instant of a
nonadiabatic transition. Photoelectron spectroscopy
enables selective observation of valence electrons thatplay central roles in nonadiabatic transitions. The
angular distribution of a photoelectron is a particularly important observable because a nonadiabatic
transition changes the three-dimensional shape of an
occupied electron orbital and aﬀects the photoelectron wavefunction. Ultrafast photoelectron imaging
visualizes the photoelectron speed and angular
distributions as a movie with a femtosecond time
resolution and enables identiﬁcation of nonadiabatic
transitions.56)–58)
How are the photoelectron kinetic energy andangular distributions expressed? In the absence of an
external electromagnetic ﬁeld, molecules in the gas
phase have random alignment of the molecular axes.
One-photon absorption induced by a linearly polarized pump pulse promotes molecules to an excitedstate whose transition dipole moments are favorablyaligned with the electric ﬁeld vector of the pumppulse. The probe pulse induces ejection of a photoelectron from this aligned ensemble of molecules inthe excited state. Based on these two optical steps,
the ﬁnal photoelectron kinetic energy and angular
distribution can be expressed byðE; tÞ
f1 þ 2 ðE; tÞP2 ðcos Þ4þ 4 ðE; tÞP4 ðcos Þg

½2

for a probe pulse whose linear polarization is parallelto that of the pump pulse. In this expression, E is
the photoelectron kinetic energy, 3 is the electron
ejection angle relative to the laser polarization
direction, and t is the pump–probe time delay.
Pn(x) is the nth-order Legendre polynomial. <(E, t)
represents the photoelectron kinetic energy distribution (i.e., the photoelectron spectrum). The anisotropy parameters, O2(E, t) and O4(E, t), contain
information about the spatial distribution of an
ionized orbital. Two-dimensional maps of <(E, t)
and O2(E, t) against time and energy are the key
observables for analyzing nonadiabatic dynamics.
Ultrafast photoelectron imaging combines theultrafast pump–probe method and two-dimensional
position-sensitive detection of electrons. An expanding distribution of photoelectrons is created when alaser pulse ionizes molecules in the vacuum chamber.
Photoelectron imaging accelerates these electrons
in a static electric ﬁeld and projects them onto a
two-dimensional position-sensitive detector (Fig. 7).
Since this method enables the total speed andangular distributions of photoelectrons to be simultaneously measured on a shot-to-shot basis, it
achieves unprecedentedly high eﬃciency and accuracy. Ultrashort UV pulses required for this experiment can be generated by nonlinear optical processes
in rare gases. Since a gas has a small nonlinear
susceptibility, a long interaction length between laser
pulses and a gas medium is required to realizeeﬃcient wavelength conversion. To obtain a suﬃciently long interaction length, we employ ﬁlamentation propagation of intense laser pulses; when an
intense laser pulse propagates through a gas, it is selffocused by the optical Kerr eﬀect and defocusedby the plasma created by ionization of the gas.
Consequently, a laser beam propagates through a gas
while maintaining a small beam diameter for a lengththat is considerably greater than the confocal parameter.59) We introduce 775 and 388 nm pulses to
Ne (0.08 MPa) and induce cascaded four-wave mixingof 2!!388  !!775 ! !!264 and !!264 þ !!388 !!775 ! !!198 to generate 264 and 198 nm pulses(Fig. 7).60) These pulses are separated and compressed using a grating-based compressor to achievepulse durations of 14 and 17 fs, respectively.60),61)
We excited pyrazine molecules in a supersonicmolecular beam using the 264 nm pulse and sub-No. 1]Visualization of chemical reaction dynamics: Toward understanding complex polyatomic reactions

7

Fig. 7. Schematic diagram of ﬁlamentation four-wave mixing to generate sub-20 fs deep UV pulses and our photoelectron imaging setup.

sequently ionized them with the 198 nm pulse.
Figure 8b shows the photoelectron intensity observed
as a function of the pump–probe time delay. The
intensity decays rapidly in the ﬁrst 100 fs andsubsequently plateaus.62),63) This time proﬁle iswell explained by assuming rapid decay of optically
excited S2 (red) and formation of S1 (blue) byinternal conversion from S2. In the negative delay
time range, the 198 nm pulse excites pyrazine to S3
and the 264 nm pulse ionizes (green); since this isunrelated to the present discussion, we neglect it
here. The S2!S1 internal conversion time constants
are determined as 23 ’ 4 fs for pyrazine-h4 and20 ’ 2 fs for pyrazine-d4. Close examination reveals
that the signal oscillates in the plateau region. The
Fourier transform of this feature (t > 50 fs) exhibits afrequency component of 560 ’ 40 cm!1 for pyrazine
and 550 ’ 40 cm!1 for pyrazine-d4. Based on thesefrequencies, the oscillation is assigned to vibrational
wave packet motion of mode 6a in S1 created byinternal conversion.
We now examine the kinetic energy distributionof photoelectrons. Figure 8a shows a time–energy
map of <(E, t). The colors yellow, red and violet
indicate decreasing intensities. <(E, t) exhibits nomarked change in distribution; the intensity diminishes gradually from a low to high kinetic energy at
all times. The kinetic energy distribution does notchange on internal conversion, because photoionization predominantly occurs as D0(n!1)AS1(n, :*) andD1(:!1)AS2(:, :*) (see Fig. 9) and these vertical
ionization energies are almost identical.64),65) In otherwords, removal of a :* electron always requires thesame energy.

How is the photoelectron angular distribution?
Figure 8c shows a time–energy map of O2(E, t);
the positive (blue-green) and negative (red) values
respectively indicate preferential ejection of anelectron parallel and perpendicular to the probe laser
polarization. The most distinctive feature in O2(E, t)
is the sudden change at ca. 30 fs; this notablechange in O2(E, t) is unambiguous evidence for
S2!S1 internal conversion. (The inﬂuence of molecular rotation can be neglected on this time scale.)
After 30 fs, O2(E, t) does not change, because theelectronic character remains (n, :*). The lack ofrestoration of the (:, :*) character is related to thephotoexcitation energy; in our experiment, photoexcitation of pyrazine near the S2 origin preparesa wave packet with a small vibrational excess energy
in S2. Consequently, if the vibrational energy ﬂows
into various modes in S1, the wave packet has noopportunity to return to the Franck–Condon regionin S2.
Ultrafast S2!S1 internal conversion of pyrazine
has been considered as a two-state problem via aconical intersection. The above-described results ofultrafast photoelectron imaging are consistent withthis well-accepted picture. On the other hand, recent
quantum chemical calculations suggest that opticallydark electronic states of 1Au and 1B2g are also
involved in the ultrafast internal conversion dynamics of pyrazine and that a fraction of the populationin S2 ﬂows into these states.40),66) The proposedmechanism is currently being investigated in ourlaboratory by ultrafast photoelectron imaging using avacuum UV probe pulse (160 nm).

T. SUZUKI

8[Vol. 89,

Fig. 9. Photoionization schemes of pyrazine within the framework of the frozen-core approximation.

Fig. 8. (a) Time-evolution of photoelectron kinetic energy
distribution, <(E, t). (b) Temporal proﬁles of total photoelectron
signals in (1 D 1B) TRPEI of pyrazine-h4. The observed data are
well explained by three components: single-exponential decay ofS2 (red); corresponding increase in S1 (blue) in the positive timedelay; and single-exponential decay of S3 (green) in the negativetime delay. The ﬁtting result is shown by the solid line. (c) Timeevolution of photoelectron angular anisotropy parameter
O2(E, t).

3. New challenge: ultrafast photoelectron
spectroscopy of nonadiabatic
dynamics in liquids

The preceding two sections discussed chemical
dynamics in the gas phase. The study of gas-phasereactions provides the basis for understanding more
complex dynamics in condensed phases. The greatest
diﬀerence between gas and condensed phases is thatsolvent eﬀects play important roles in the latter
phases. Despite not fully understanding the mechanisms involved, chemists have utilized solvent eﬀects
for centuries to alter potential energy surfaces
and reaction dynamics. It is thus exciting to apply
ultrafast photoelectron spectroscopy to liquids toinvestigate static and dynamic solvent eﬀects

and nonadiabatic processes, particularly in aqueous
solutions.
Siegbahn67) pioneered photoelectron spectroscopy of liquids in the 1970s. The technical problem tointroduce volatile liquids into a photoelectron spectrometer was solved to a large extent by a liquidbeam technique developed by Faubel and coworkers68) in the 1980s. Winter and coworkers69)
performed photoelectron spectroscopy of liquidbeams with third-generation synchrotron radiation.
Kondow and Mafune combined a laser with liquidbeams to perform photoionization mass spectrometry
in 1992.70) It took nearly two decades before ultrafast
photoelectron spectroscopy of liquids was realized
in 2010.71)–73) This section describes this very new
research ﬁeld.
An electron passing through a liquid is scatteredby solvent molecules. This scattering determines
the escape depth of a photoelectron (and hence theprobing depth of photoelectron spectroscopy). Therefore, the inelastic mean free path of an electron in
the bulk is a very important quantity. However, theinelastic mean free path is not well understood forany liquid. It is thus necessary to speculate about it
based on those in solids. Experimental studies of solidmaterials show that the mean free path becomesminimal (<1 nm) for kinetic energies in the range
50–100 eV.74) Inelastic scattering of an electron in
amorphous ice indicates that electron energy losses
greater than 1 eV rarely occurs for kinetic energies
smaller than the HOMO–LUMO gap (97 eV) of bulk
water.75) This is because such low-energy electrons
cannot excite the valence electrons of water andNo. 1]Visualization of chemical reaction dynamics: Toward understanding complex polyatomic reactions

Fig. 10. Schematic energy diagram of time-resolved photoelectron spectroscopy of charge-transfer-to solvent reaction. Reproduced with permission from Ref. 94.

hence interact only with phonons. Since electron–
phonon scattering has a considerably smaller crosssection than electron–electron scattering, the inelastic mean free path (and hence the probing depth ofphotoelectron spectroscopy) increases with decreasing kinetic energy. (Another extreme case is hardX-ray photoelectron spectroscopy, which is also bulk
sensitive.)
Using ultrafast photoelectron spectroscopy ofliquids, we studied the charge-transfer-to-solvent(CTTS) reaction from I!(aq) to bulk water (seeFig. 10).76)–78) A hydrated I! atom has an electron
binding energy of 8.0 eV and its ground state lies inthe band gap of bulk water; the electron binding
energy of bulk water is 11.2 eV.79) The UV absorption
maximum of I!(aq) is at 225 nm, corresponding to
excitation to the metastable (CTTS) electronic state.
The CTTS state rapidly undergoes exothermic
electron transfer to bulk water. When the excess
electron migrates into bulk water, a local hydrogenbonding network around the I atom and the excess
electron change drastically due to reorientation ofwater molecules; I! and an excess electron cloud arehydrophilic, whereas a neutral I atom is hydrophobic.
Thus, the CTTS dynamics is strongly coupled withthe solvation dynamics. The I atom has no internal
degrees of freedom besides electronic motion, making
it a suitable solute for investigating the CTTS
dynamics. We excited I!(aq) with the 226 nm pumppulse and photodetached the excess electron witha time-delayed 260 nm probe pulse. A hemisphericalelectron energy analyzer with extensive diﬀerential
pumping was employed.80)

9Fig. 11. Photoelectron signal intensity as a function of pump–probe time delay observed for charge-transfer-to-solvent reaction
from I! to bulk water: the sample solutions were 0.1 M aqueous
NaI solution in H2O (black) and D2O (blue). The schematicenergy diagram of the experiment is shown in Fig. 10. The crosscorrelation of the pump and probe laser pulses was ca. 300 fs.
A hemispherical electron energy analyzer was used to measure
the photoelectron spectrum at each time delay and the spectrum
was integrated over the photoelectron kinetic energy to obtaineach data point. From Ref. 94.

Figure 11 compares the photoelectron signal
intensity, I(t), observed for aqueous NaI solutions of
H2O and D2O as a function of the pump–probe delay
time. The ﬁgure indicates a clear isotope eﬀect: thephotoelectron intensity in the ﬁrst few picoseconds
diminishes much faster in D2O than in H2O, while theopposite is true in the subsequent 30 ps. Anotherisotope eﬀect is seen in Fig. 12 that shows <(E, t) for
solutions in H2O and D2O. <(E, t) has similar generalfeatures for H2O and D2O; however, the spectral
width of <(E, t) is narrower and its intensity
diminishes faster in D2O than in H2O. The timedependences of the photoelectron intensity andkinetic energy distribution can be explained by themechanism illustrated in Fig. 13. After photoexcitation of I! to a metastable state, the excess electron
distribution becomes asymmetric in the hydration
shell and separated from a neutral I atom. Calculations for Cl! and water suggest that attractive
interactions occur between a neutral atom and ahydrated electron;81) a similar attraction is anticipated between an I atom and a hydrated electron.
The hydrated electron and I atom are ﬁnallyseparated by diﬀusion. The photon energy of ourprobe laser pulse is higher than the binding energiesof the CTTS state, the contact pair state, the solvent
separated state, and a free hydrated electron, but it islower than the ground state of I!(aq). Therefore, the10T. SUZUKI[Vol. 89,

Fig. 12. Two-dimensional false-color map for photoelectron kinetic energy distributions measured for charge-transfer-to-solvent reaction
from I! to bulk water for various pump–probe time delays. The schematic energy diagram of the experiment is shown in Fig. 9.
Experimental data for 0.1 M aqueous NaI solution in (a) H2O and (b) D2O. (c) and (d) show the results of global ﬁtting ofexperimental data using a kinetic model. From Ref. 94.

Fig. 13. Graphical presentation of our kinetic model with some representative time constants for the charge-transfer-to-solvent reaction
from I! to bulk water.

disappearance of the photoelectron signal in Fig. 11
is ascribed to internal conversion to the groundelectronic state of I!(aq); i.e., geminate recombination of an electron and an I atom. In the initial stepsof the CTTS reaction, H2O has a faster librational
response than D2O. Consequently, the quantum yieldof internal conversion and the loss of electron signal
are greater in D2O during the early stages of theNo. 1]Visualization of chemical reaction dynamics: Toward understanding complex polyatomic reactions

reaction. On the other hand, internal conversion isnearly two times faster in H2O than in D2O during
the later stages of the CTTS reaction.
The CTTS reaction for bulk water ultimately
creates hydrated electrons,56),71)–73),82)–94) which are
the most important transient species in radiation
chemistry and biology. The electron binding energyof a hydrated electron has been speculated to be
3.3 eV, although no direct measurement has been
performed. Our photoelectron spectroscopy of liquids
has determined that the electron binding energy of ahydrated electron is 3.4 eV and that the energies ofsolvated electrons in methanol and ethanol are 3.5
and 3.3 eV, respectively.66),72),73),94),95) Interestingly,
the electron binding energy of a hydrated electron
agrees with a previous estimate (3.3 eV) obtainedby extrapolating the binding energies in negatively
charged water clusters.84) On the other hand, theelectron binding energy of a solvated electron inmethanol is much greater than the estimate(2.6 eV) from corresponding cluster values.96) The
cause of this interesting diﬀerence is currently being
investigated.
Ultrafast photoelectron spectroscopy of liquids isa very young ﬁeld and there are many unsolved
problems for this methodology. On the other hand,
it is promising for elucidating electronic dynamics
in solution and providing important new knowledge
for chemistry, physics and biology.
Acknowledgments

I would like to thank H. Kohguchi, Y. Ogi, Y.
Suzuki, T. Fuji and T. Horio for their contributions
to the research described in this paper.

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(Received June 29, 2012; accepted Oct. 12, 2012)No. 1]Visualization of chemical reaction dynamics: Toward understanding complex polyatomic reactions

ProﬁleToshinori Suzuki was born in 1961. As a freshman in Tohoku University he wasinspired by quantum chemistry lectures given by the late Prof. Yoshito Amako, and hedecided to study physical chemistry. He graduated from the university in 1984, andreceived the doctoral degree from the same university in 1988 under the supervision ofProf. Mitsuo Ito. He immediately joined the research group of Prof. Eizi Hirota at theInstitute for Molecular Science as a technical associate and later became a research
associate. During this period, he investigated O(1D) reaction with methane by infrared
diode laser spectroscopy, and he decided to develop a crossed-beam ion imaging
experiment for studying polyatomic reactions. In order to realize it, he resigned Institutefor Molecular Science and joined Cornell University (Prof. Paul Houston) and the
University of California, Berkeley (Prof. Yuan T. Lee) as a JSPS fellow for oversea studies. In 1992, he wasappointed again by the Institute for Molecular Science as an associate professor and returned to Japan to start his
research group. He initiated a crossed-beam imaging experiment and also realized femtosecond photoelectron
imaging. In 2001, He moved to RIKEN to be a chief scientist and extended his research further. The crossed-beamimaging experiment on O(1D) reaction with methane was realized ﬁnally in 2008, 20 years after his ﬁrst planning.
He started the research project on chemical reaction dynamics in water using the advanced facilities of SPring-8
and SACLA (x-ray free electron laser), which is on going. In 2009, he became a professor of physical chemistry at
Kyoto University to inspire, and be inspired by, young students. He received Broida Award, IBM Science Award,
JSPS Prize and Commendation for Science and Technology by MEXT. He served as a president of Japan Society
for Molecular Science for 2010–2012.

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