22 Phys. Chem. Chem. Phys., 2012, 14, 22–34 This journal is c the Owner Societies 2012

Cite this: Phys. Chem. Chem. Phys., 2012, 14, 22–34

On the nature and signatures of the solvated electron in water

B. Abel,*aU. Buck,

bA. L. Sobolewski

cand W. Domcke

d

Received 3rd June 2011, Accepted 9th September 2011

DOI: 10.1039/c1cp21803d

The hydrated electron is one of the simplest chemical transients and has been the subject of

intense investigation and speculation since its discovery in 1962 by Hart and Boag. Although

extensive kinetic and spectroscopic research on this species has been reported for many decades,

its structure, i.e., the dominant electron–water binding motif, and its binding energy remained

uncertain. A recent milestone in the research on the hydrated electron was the determination of

its binding energy by liquid-jet photoelectron spectroscopy. It turned out that the assumption of a

single electron binding motif in liquid water is an oversimplification. In addition to different

isomers in cluster spectroscopy and different transient species of unknown structure in ultrafast

experiments, long-lived hydrated electrons near the surface of liquid water have recently been

discovered. The present article gives an account of recent work on the topic ‘‘solvated electrons’’

from the perspectives of cluster spectroscopy, condensed-phase spectroscopy, as well as theory.

It highlights and critically discusses recent findings and their implications for our understanding

of electron solvation in aqueous environments.

I. Introduction

The solvated electron is one of the simplest reactive species in

chemistry and biology. Electrons trapped in liquid ammonia,

water, alcohols, amines and other polar liquids have been of

interest in chemistry since a long time. The hydrated electron,

in particular, is a fundamental transient species in the radiation

chemistry of water. Initially, it has been identified as a second

reducing agent (besides the hydrogen atom) in the pulse radiolysis

of pure water, which is formed with a higher yield than the

H atom.1 The absorption spectrum of this species, centered at

720 nm, was discovered by Hart and Boag in 1962.2 This intense,

broad and characteristically asymmetric absorption spectrum,3

which is shown in Fig. 1, has become the most characteristic

feature of the hydrated electron.1

Prominent features of the hydrated electron are its finite

lifetime (of the order of microseconds in pure water), its

exceptionally high rate of diffusion and its high reactivity.1

The kinetic rate constants of numerous reactions involving the

hydrated electron have extensively been investigated and

tabulated.4 Apart from its role in the radiation chemistry of

water (e.g., in nuclear-reactor and nuclear-waste technology5),

the hydrated electron is also formed by ionizing radiation in

biological tissues and has a significant potential for the

damage of DNA.6 From the fundamental point of view, the

hydrated electron has fascinated generations of chemical

physicists as the simplest possible model of a quantum particle

interacting with a classical thermal environment.7–10

The present article is intended to give a timely account of

recent developments in the investigation of the hydrated

electron, in particular by time-resolved photoelectron emission

spectroscopy, by the spectroscopy of size-selected aqueous

clusters, as well as with computational methods. The paper

is organized as follows: after a short general introduction we

discuss theoretical models of the solvated electron in the

condensed phase. Recent insights obtained by the spectroscopy

of negatively charged water clusters, water clusters doped by

alkali metal atoms, as well as acidic water clusters are surveyed

in Section III. In Section IV, we summarize recent experiments

on the photoelectron spectroscopy of hydrated electrons in

Fig. 1 Absorption spectrum of the hydrated electron in bulk water

under ambient conditions (adapted from ref. 113).

aWilhelm-Ostwald-Institut fur Physikalische und TheoretischeChemie, Universitat Leipzig, D-04103 Leipzig, Germany.E-mail: abel@rz.uni-leipzig.de

bMax Planck Institut fur Dynamik und Selbstorganisation,D-37073 Gottingen, Germany

c Institute of Physics, Polish Academy of Sciences, PL-02668 Warsaw,Poland

dDepartment Chemie, Technische Universitat Munchen,D-85747 Garching, Germany

PCCP Dynamic Article Links

www.rsc.org/pccp PERSPECTIVE

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This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 22–34 23

liquid water beams that yield reliable binding energies of the

solvated electron—one of the missing links in the investigation

of the solvated electron in water. In Section V we critically

discuss what is known and what is not yet known about this

species. We close with conclusions and outlook.

II. Models of the structure and dynamics of the

hydrated electron in the liquid phase

Many decades after its discovery, the hydrated electron is still

not fully characterized in terms of structural and spectroscopic

properties. The concept of an excess electron which is trapped

in a cavity or void in the hydrogen-bonded network of liquid

water was first proposed by Stein, Platzmann and Weiss already

in the 1950s.7–9 The first attempts towards a quantitative

description of the properties of the hydrated electron were based

on the model of a spherical charge distribution interacting with a

polarizable dielectric continuum.10

Alternative to continuum models, so-called molecular-field

models were proposed, where it is assumed that the excess

electron is surrounded and solvated by a fixed number of

water molecules in a specific configuration, e.g. in tetrahedral11

or octahedral12 cages. A proposed geometrical structure of the

hydrated electron, derived from electron spin resonance data in

aqueous glasses, is shown in Fig. 2. A comprehensive review of

the early continuum and molecular-field models of the hydrated

electron in bulk water was given by Feng and Kevan.13

Since the 1980s, the so-called cavity model of the hydrated

electron has been elaborated in considerable detail by Rossky,

Borgis, Schwartz and coworkers, treating the excess electron

as a localized quantum particle in the environment of classical

rigid water molecules.14–18 Using partially empirical electron–

water pseudopotentials, extensive numerical simulations of the

stationary and transient absorption spectra of the hydrated electron

were performed. While the position and width of the stationary

absorption spectrum could be reproduced, the simulations failed to

account for the characteristic asymmetry and the extended ‘‘blue

tail’’ of the absorption profile (see Fig. 1).14–18 Very recently, it has

been demonstrated that the blue tail can be described in simulations

with electronically polarizable water molecules.19 On the other

hand, a recent simulation employing an improved electron–

water pseudopotential suggests a model in which the excess

electron resides in a region of enhanced water density rather

than in a cavity.20 Despite enormous progress in ab initio

quantum chemistry and in the available computer power since

the initial conception of the cavity model, a quantitative

theory of the structure and the properties of the hydrated

electron in bulk water is still elusive.19–21

While the cavity model has been the consensus picture of the

hydrated electron since six decades, alternative proposals were

sporadically made. In the 1980s, Robinson and collaborators

as well as Tuttle and Golden called the cavity model into

question and proposed that molecular solvent–anion complexes

or radical–anion complexes, such as hydrated H3O–OH�,

shown in Fig. 3, could be the carriers of the spectroscopic

and chemical properties of the hydrated electron.22,23 Muguet

and Robinson emphasized the itinerant character of the H3O

radical in water which might explain the exceptionally high

mobility of the hydrated electron.24 Sobolewski and Domcke

pointed out (see also Section III) that the hydrated hydronium

(H3O) radical is a charge-separated species in water and as such

very effectively solvated,25–27 see also ref. 28. The calculated

geometrical structures and the molecular orbitals of the unpaired

electron of H3O(H2O)3n, n=0, 1, 2, clusters are shown in Fig. 4.

It is seen that the charge density of the unpaired electron detaches

from the H3O+ cation with increasing cluster size. Moreover,

the ab initio calculated electronic and vibrational spectra of the

hydrated H3O radical exhibit striking similarities with the

spectral signatures of the hydrated electron.25 For example,

the blue tail of the absorption spectrum is readily explained by

such molecular models of the hydrated electron.26,27

In addition to pulse radiolysis with X-rays or electrons, the

hydrated electron can be generated by single-photon or two-

photon excitation of water,29–35 by photodetachment from

anions in aqueous solution36–40 or by photoexcitation of organic

Fig. 2 Proposed geometrical structure of the solvated electron in

aqueous glasses, derived from electron spin resonance data (adapted

from ref. 13).

Fig. 3 ‘‘Intuitive’’ structure, (H3O–OH�), of the hydrated electron in

bulk water, postulated by Hameka, Robinson and Marsden (adapted

from ref. 22).

Fig. 4 The singly occupied highest molecular orbital of H3O (left),

H3O(H2O)3 (middle) and H3O(H2O)6 (right) clusters, calculated at the

restricted open-shell Hartree–Fock level.25 The colors green/yellow

code the sign of the wavefunction. Dotted lines indicate hydrogen

bonds.

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24 Phys. Chem. Chem. Phys., 2012, 14, 22–34 This journal is c the Owner Societies 2012

chromophores with acidic groups, such as indole or phenol.41–46

The resonant photodetachment of electrons from anions in

dilute salt solutions is the most convenient method of genera-

tion of hydrated electrons, since the absorption cross sections

of anions in water are large, the excitation energy is convenient

for lasers, and the quantum yields are high.40 Despite decades

of intensive research, the mechanisms involved in photo-

ionization in aqueous solutions remain controversial. It has been

pointed out by Bradforth and coworkers that electron ejection

from neat water and photodetachment of anions in water may

be quite distinct processes.47 The mechanisms of relaxation

towards the fully equilibrated hydrated electron are complex

and depend strongly on the excitation process. In the case of

anion photodetachment, it is assumed that charge-transfer-

to-solvent (CTTS) states are transiently populated.40,47 In the

case of UV two-photon excitation of neat water, a number of

intermediate species (‘‘wet electrons’’) have been postulated

for the interpretation of transient spectra.48 The existence of

such intermediate species and their association with structural

models remain controversial issues.49–53

The radiationless decay of the s - p excited state of the

hydrated electron has been another hotly debated issue since

several decades. The ongoing controversy concerns the mechanism

by which the electron relaxes to equilibrium after impulsive

photoexcitation. While the femtosecond pump–probe data

obtained by several groups are similar, the controversies arise in

the interpretation. It is disputed whether the relaxation occurs by

rapid nonadiabatic radiationless decay to the electronic ground

state, followed by comparatively slow (picosecond) ground-state

cooling, or by relatively slow nonadiabatic excited-state decay,

followed by rapid equilibration in the ground state.54–57

Quantum-classical molecular simulations can explain either

scenario, depending on the electron–water pseudopotential

and the nonadiabatic surface-hopping algorithm.16,20,58–61

III. The cluster approach: models of the structure

and the dynamics of electrons in finite-size aqueous

clusters

Already very early, experiments with clusters played a crucial

role in elucidating the structure and the spectroscopic proper-

ties of the hydrated electron. Negatively charged water cluster

anions are natural finite-size models of the hydrated electron.

The size selection is no problem and once it was discovered

how to produce these clusters,62,63 measurements of the binding

energies of the excess electron were carried out for large64,65 and

small clusters.66 The size dependence of the electron binding

energy followed the predicted behaviour.67,68 Based on the

picture of spherical clusters of radius R, a plot of the measured

vertical binding energies versus 1/R (and thus n�1/3, where n is

the cluster size) gives straight lines with negative slopes for the

cluster anions. Theoretical estimates of the slope can be derived

from dielectric continuum theory. An important issue, which is

often overlooked in using this type of diagram, is the fact that

this theory, which is based on bulk matter concepts, does not

distinguish between surface and interior states of the electron.67

The extrapolation of the data for anionic water clusters to the

bulk value (n - N), reported by the Bowen group,65,69,70

gives 3.3 eV. These data are shown in Fig. 5 in blue colour.

The experimental results were accompanied by calculations

which dealt with the structures and energetics of these clusters.

For large clusters in the size range n = 8–128, the quantum

path-integral molecular dynamics method with a pseudo-

potential for the electron–water interaction was used.71,72 The

calculations predicted the existence of diffuse surface-bound

states as well as more compact interior states of the excess

electron. For small and intermediate sized clusters (n = 8–32),

the energetically favoured localisation of the excess electron is

a surface state, while for larger clusters (n = 64, 128), the

internal localisation of the electron is preferred. The calcula-

tion of the electronic absorption spectra gave good agreement

with the experimental data for the surface states.73

Recent extensive photoelectron emission and laser-spectro-

scopic investigations of intermediate-size and large water

cluster anions by the groups of Bowen, Johnson, Zewail and

Neumark revealed a considerably more complicated picture.69,74–78

A major breakthrough in this field was the discovery of new

classes of cluster isomers by the Neumark group when the

conditions of producing the clusters were changed.74,79,80 The

photodetachment spectra obtained for cluster sizes n = 11–200,

produced at normal pressures of 2.1 bar of the water–argon

mixture, were similar to those obtained by Coe et al.65 When

the backing pressure was increased to 4.8 bar, however, a

much lower value of the vertical binding energy was obtained.

The complete results are plotted in Fig. 5 in red colour.

The two distinct groups of isomers are labelled I and II.

These findings have been confirmed in an experiment with

larger photon energy (4.7 eV instead of 3.1 eV).80 The ex-

planation of these results is as follows. In the case of expan-

sions at normal pressure (group I), the electrons are attached

to relatively warm clusters with high internal energy, which

readily leads to the lowest energy configurations by solvent

rearrangement. The expansions at high pressure (group II), on

the other hand, produce rather cold clusters with small

reorganisation energies, which carry the excess electron in a

metastable state, preferentially at the surface of the cluster.

Fig. 5 Vertical binding energies of (H2O)n� clusters (blue: ref. 65;

red: ref. 74; green: ref. 81) as functions of n�1/3. The light blue points

mark the binding energies of interior and surface electrons in the

liquid.129 The violet triangles are based on recent calculations.144

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This interpretation was confirmed by a very recent measure-

ment of the vertical binding energies of very cold clusters.81 In

this experiment, heating occurred between the production and

the thermalisation of the clusters to low temperatures, so that

the clusters of class II are nearly completely suppressed and

only isomers of class I are observed. The photoelectron spectra

consist of up to three peaks, the energies of which are plotted in

Fig. 5 in green colour. Very recent DFT-based molecular

dynamics simulations for n = 32 demonstrated that in cold

clusters the electron becomes trapped in a metastable state with

comparatively low binding energy,82 while electron attachment

to warm, liquid clusters results in an equilibrated and compara-

tively strongly bound species.83,84 Since calculations usually

predict that internally solvated electrons are more strongly

bound than those at the surface,71,85 the class I isomers are

assigned to internally bound electrons and class II isomers to

surface-bound electrons. We note, however, that the calculated

binding energies do not agree with the measurements, being in

both cases too large. Interestingly, the measured electronic

absorption spectra86 are better reproduced by the surface

isomers in the computational simulations up to n = 50.85

The experimental results were complemented by the measure-

ment of electronic relaxation time scales75,76 and vibrational

spectroscopy of the OH stretching band.77 In the latter case,

the spectra exhibit the double-acceptor motif for small clusters

of the isomer class I.87 From the variety of experimental

results and the different data sets for the class I isomers, it is

difficult to pin down exactly the group of clusters for

which the data extrapolate correctly to the bulk values. Recent

Born–Oppenheimer (BO) molecular-dynamics simulations for

(H2O)32� at T = 350 K, based on DFT with the PBE

functional, showed a strong correlation of the binding energy

with the degree of localisation of the electron, rather than with

the interior or surface character.88

Extensive simulations of (H2O)n� clusters of various sizes

and at various temperatures with established mixed-quantum-

classical methods did not lead to definitive structural assign-

ments.85,89–91 Ab initiomolecular dynamics calculations provided

evidence that the excess electron is located at the surface of the

cluster, even if it is initially placed in the interior.88 Although

numerous ab initio calculations with self-consistent-field (SCF),

second-order Møller–Plesset (MP2) or DFT methods have been

performed for selected structures of (H2O)n� clusters, it is not

possible to explore systematically the isomers due to the huge

number of possible structures.92–96 The question of the electron

binding motif of intermediate-size water cluster anions and its

extrapolation to bulk water thus remains unsettled.

An alternative approach towards the modeling of the

hydrated electron by finite-size clusters is inspired by the

well-known beautifully blue solution of metallic sodium in

liquid ammonia.97 In the pioneering work of Schulz, Hertel

and coworkers as well as Fuke and collaborators, the size-

dependence of the vertical ionization energy, the vertical

electronic excitation energy and the excited-state lifetime of

M(H2O)n (M= Li, Na, K) clusters were determined.98–103 The

electronic excitation energy of Na(H2O)n clusters exhibits a

significant and non-monotonic dependence on the cluster

size.101 The excited-state lifetimes of Na(H2O)n clusters are

systematically shorter than those of (H2O)n� clusters.103 In IR

spectra of size-selected sodium-doped water clusters, finger-

print OH vibrations have been detected which indicate a

delocalized electron at the surface of the clusters.104,105

The results of the Hertel group for the ionization potentials

(IPs) of Na(H2O)n clusters were very surprising. Only the first

four values of the measured IP decrease, as expected, in the

n�1/3 plot, while the IP stays constant at a value of 3.17 eV for

all larger clusters.99,100 Very similar results were obtained for

Li(H2O)n clusters with an IP of 3.12 eV and Cs(H2O)n clusters

with an IP of 3.10 eV for n Z 4.98,106 There were numerous

speculations on the explanation of the unexpected constant

behaviour of the IP of alkali–water clusters. In one of the first

experimental papers,99 the transition from a one-center to a

two-center localization of the electron was discussed, taking

up an idea of Jortner.68 The earliest calculation on sodium–

water clusters, based on spin density functional theory, confirmed

indeed the complete separation of the electron from the Na+

cation from n Z 4 onwards and its delocalization over the water

molecules.107 As for the reason of the constant ionization potential,

a compensation of forces was suggested. Ab initio calculations at

the UMP2 level were also carried out for small Na and Li water

complexes.108,109 The calculated IPs for the interior complexes are

lower than the experimental values, while the calculated IPs of the

surface complexes are higher than the measured values. In a more

recent publication, the constant IP was explained by an auto-

ionization process after a transition to high Rydberg states and

the release of considerable vibrational reorganisation energy.110

Very recently, a second class of isomers has been observed

also for Na-doped water clusters in a special experimental

set-up which is capable of generating molecular beams with high

abundances of large clusters.111 These results are displayed in

Fig. 6. One group with the already known IP of 3.2 eV (group I)

has been extended up to the size n = 500, and the existence of a

second group of isomers with the lower IP of 2.8 eV has been

established (group II). In the second group, the IP decreases in

the range 11r nr 15 and then stays constant, as was found for

class I. The existence of two classes of isomers in Na(H2O)nclusters calls for a comparison with the data obtained for the

water cluster anions, which are included in Fig. 6. Apart from

the different asymptotic behaviour, there are other remarkable

differences. The bifurcation of the IPs into two branches occurs

for the Na-doped clusters at smaller sizes and for the same

expansion conditions. The difference of the two extrapolated

electron binding energies is smaller for the Na(H2O)n clusters

(0.4 eV) than for the largest (H2O)n� clusters (1.05 eV). For the

Na(H2O)n clusters, the lower IP and the larger critical size of

group II indicate that the charge-separation distance of the

Na+– e� ion pair should be larger in the isomers II. This is

indeed confirmed by a simulation of the structures, using ab initio

molecular dynamics and the modelling of the ionization spectra

with the PMP2 method.111 For n = 15, two isomers could be

distinguished with IPs of 3.1 eV and 2.6 eV, respectively, close to

the measured values. According to the simulations, the electron is

in both cases distributed over the water molecules at the surface

of the cluster. The isomers differ, however, in the Na+– e�

distance, which is 3.28 A for the group I clusters and 4.86 A

for the group II clusters. It was concluded that one isomer

corresponds to a contact ion–electron pair and the other isomer

to a solvent-separated electron–ion pair.111

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The hydronium (oxonium) radical, H3O, is isoelectronic with

the sodium atom. It is thus not surprising that the H3O radical

dissociates into an H3O+ cation and a solvated electron in water

clusters,25,28,112 as has been found for alkali–water clusters.

There is a fundamental difference, however: while the spherical

alkali cations can be solvated by numerous conformations of the

hydrogen-bonded network of water, the H3O+ cation is a rather

rigid trigonal pyramid which forms exceptionally strong and

directed hydrogen bonds with the first solvation shell (the

so-called Eigen structure of protonated water). The H3O+

cation thus induces, by its rigid geometry, a microcavity in the

hydrogen-bond network of water.25 It has been argued that

the structural rigidity of the (H3O+)aq(e

�)aq ion pair could be

the origin of the surprising shape stability113 of the absorption

spectrum of the hydrated electron.

While H3O(H2O)n clusters are difficult to prepare directly

(through charge exchange of H3O+(H2O)n clusters with alkali

atoms114,115), their transient existence has been inferred indirectly

through the photodissociation of hydrogen halide (HX)molecules

in water/ice nanoparticles.116–118 The solvated H3O radical is

generated in these experiments through CTTS excitation of the

charge-separated H3O+X� species and is detected by the

characteristic translational energy distribution of the H-atoms

arising from the dissociation of metastable H3O(H2O)n clusters.

The experiments on the photodissociation of pure and doped

water clusters were carried out in the standard molecular beam

arrangement.119 The HX(H2O)n clusters were generated by

adiabatic expansion and then dissociated by UV laser photons

at 193 nm or 243 nm. The arising H-atoms were observed by

resonance-enhanced multiphoton ionisation at 243 nm. The

H-atom fragment kinetic energy was detected in a time-of-flight

spectrometer and is the main observable in these experiments. The

photodissociation of hydrogen halides on Arn clusters, in which

no ion-pair separation takes place, serves as a reference.120,121 The

results for HClArn clusters are shown in Fig. 7.120 The narrow

peak at zero kinetic energy corresponds to the caged fragments,

while the peaks at higher energy result from the direct cage exit of

the fragments, reflecting the two spin–orbit states of the Br

radical. The results for the HX(H2O)n systems look completely

different, as shown in Fig. 8.117,122 They exhibit only slow

fragments with energies below 0.5 eV. The differences between

the HXArn and HX(H2O)n kinetic-energy spectra can be

explained by postulating the formation of the H3O radical in

the HX(H2O)n clusters. The suggested mechanism is depicted

in Fig. 9.117,118,122 It starts with the acidic dissociation of the

HX molecule in the ground state, followed by laser excitation

of the X� anion to a CTTS state. The latter relaxes by a

radiationless transition to a H3OX biradical.94,122,123 The H3O

radical is metastable and will ultimately decay, yielding an

H2O molecule and the H atom which is finally detected.

Several experimental findings support this picture.

(1) Double isotope substitution: measurements with isotopic

variants of HX and H2O provide strong experimental evidence

for the hypothesis that the detected H-atom originates from

the H3O species. Assuming hydronium radical generation

from the HX and H2O molecules (without H/D scrambling),

the H3O, H2DO and HD2O radicals would be produced in

HX(H2O)n, DX(H2O)n and HX(D2O)n clusters, respectively.

Therefore, the expected ratio of H-signals from these species

would correspond to the number of H-atoms in these radicals,

i.e., 3 : 2 : 1.117,118 Table 1 summarizes the measured ratios of

the H-fragment signals from HX(H2O)n clusters to H-signals

from the deuterated species, HD2O from HX(D2O)n and

H2DO from DX(H2O)n, clusters, which are generally in good

Fig. 7 Kinetic-energy distribution spectra for the photolysis of

HBrArn clusters with n = 100.120

Fig. 8 Kinetic-energy distribution spectra for the photolysis of

HX(H2O)n clusters with n = 400.117,122

Fig. 6 Ionisation potentials and vertical electron binding energies of

the different isomers of Na(H2O)n clusters (green)99,111 and (H2O)n�

clusters (red)74 as functions of n�1/3. The light blue points mark the

binding energies of interior and surface electrons in the liquid.129

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agreement with the expected values. These ratios suggest that

statistical H/D scrambling does not occur in the clusters at

least on the timescale of the present experiment (B0.65 ms).

(2) Shape of the H-atom kinetic-energy distribution: it has

been mentioned above that the differences between the HXArnand HX(H2O)n kinetic-energy spectra indicate a mechanism of

H-fragment formation in HX(H2O)n clusters which differs

from the direct photodissociation of the HX molecule. In

addition, the spectra in Fig. 8 are the same for all three HX

molecules within the experimental error bars, which is not at

all the case for the corresponding HXArn clusters. The similarity

of these spectra suggests that the H-fragments originate from the

same species in all three systems, namely from the H3O radical,

rather than from the different HX molecules. The measured

shape agrees well with what is known of the energetics of the

decay H3O - H2O + H. The corresponding potential-energy

curve is shown in Fig. 10.25 From the top of the low barrier, the

maximal energy gain is about 0.7 eV, in good agreement with the

measurements.

(3) HCl/HBr cross-section ratio: the acidic dissociation of

HCl and HBr leads to a significant red shift of the absorption

spectrum. In fact, a new band, the CTTS band, appears. This

band can be used to detect whether HCl or HBr are acidically

dissociated under the experimental conditions. The ratio

j(HBr)/j(HCl) of the photolysis rates at the specific wave-

lengths of 191 and 243 nm is 7.4 � 1 for HX deposited on

water clusters. Calculations for small clusters give values around

2 for the dissociated structures and about 45 for the covalently

bonded structures.116 This is a clear indication that the HX

molecules are acidically dissociated, preferentially forming a

contact ion pair, since otherwise we would have observed a

pronounced dilution in the isotopically substituted experiments.

This result is in agreement with other experiments using Fourier

transform IR-spectroscopy124 and Cs+ ion scattering125 from

ice particles. The transition occurs at temperatures between

80 and 120 K, while the internal temperature of our clusters is

estimated to be 100–120 K.126

Alternative evidence of the formation of hydronium–halogen

biradicals by CTTS excitation has been obtained by Castleman

and coworkers through the detection of the released iodine

atom from HI(H2O)n clusters.127,128 Both experiments, the

detection of the released hydrogen atom and the detection of

the halogen atom, illustrate that cluster experiments can

provide information through observables which are not easily

accessible in the condensed phase, namely the kinetic-energy

distribution of the nascent products. The experimental evidence

of the central role of the H3O radical in the photochemistry of

acidic and salty water clusters has been supported by computa-

tional studies of excited-state potential-energy surfaces of

HX(H2O)n, X = Cl, Br and MX(H2O)n, M = Li, Na, K,

clusters.94,123 There is thus increasing evidence that the hydrated

H3O radical actually exists and that it can be detected spectro-

scopically—at least in (small and cold) water cluster environ-

ments. It is less clear whether it is actually the second reducing

species in the radiolysis and photolysis of liquid water, as has

been postulated by Robinson and coworkers long ago.22

IV. Photoelectron spectroscopy of hydrated

electrons in liquid jets: the missing link

Very recently four research teams reported the determination

of the vertical binding energy of the solvated electron in water,

by liquid water microjet photoelectron spectroscopy.129–132 In

all these experiments, the hydrated electrons were generated by

resonant photodetachment of electrons from an anion (I� or

Fe(CN)64�).40,133 In the first report on solvated electrons in a

water micro-jet, Siefermann et al. additionally generated

hydrated electrons by 267 nm two-photon excitation of neat

liquid water.129 In ref. 130 and 131, the solvated electron was

detected via photoemission after excitation with 274, 266, 260

or 213 nm laser light, while EUV radiation (32 nm) was

employed in ref. 129. Recently, Lubcke et al. and Tang et al.

reported femtosecond time-resolved photoelectron spectra of

the hydrated electron in a liquid jet.132,134

Instead of using UV photoelectron detachment, Siefermann et al.

directly measured vertical binding energies of hydrated electrons in

a liquid micro jet in vacuum employing a high-harmonic light

Fig. 9 Mechanism of the photodissociation of HX(H2O)nclusters.117,118,122

Table 1 Ratios of H-fragments from HX(H2O)n clusters to H-signalsfrom the deuterated species, that is, HD2O from HX(D2O)n andDH2O from DX(H2O)n

118

X Cl Br I I Expected

Wavelength, l 193 193 193 243HX(D2O)n 2.7 � 0.4 3.0 � 0.3 3.1 � 0.5 3.9 � 1.0 3.0DX(H2O)n 1.1 � 0.4 1.4 � 0.2 1.6 � 0.4 2.5 � 1.0 1.5

Fig. 10 Potential-energy function along the minimum-energy path

for H-atom detachment from the hydronium radical solvated by three

water molecules, H3O(H2O)3 (adapted from ref. 25).

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source driven by a femtosecond laser system.135,136 The special

features in their novel approach are the generation of solvated

electrons by a femtosecond pump pulse and the registration

of the photoelectron spectra using a time-delayed 38.7 eV

(32 nm) probe pulse, using a multiplex photoelectron time-of-

flight spectrometer to record the entire photoemission spectrum

for a particular time step.

In the latter study, hydrated electrons in liquid water were

generated first by 267 nm laser pulse excitation of Fe(CN)64�

anions in a 0.5 M aqueous solution of K4Fe(CN)6. The

(Fe(CN)64�)aq anion complex has a strong CTTS absorption

band near 267 nm. After absorption of UV light, the released

excited electron relaxes to an equilibrated solvated electron

within approximately 0.5 ps.37 In the experiments, the EUV

probe pulse was delayed by >100 ps relative to the CTTS

excitation in order to assure equilibration of the electron. The

intensity of the UV pump pulse was tuned to avoid

two-photon excitation of water, but was sufficiently high to

generate a significant density of hydrated electrons via CTTS

excitation of Fe(CN)64� anions. The EUV photoelectron probe

has a high surface sensitivity and selectivity. However, in this

particular experiment the detected solvated electrons were

generated mainly in the bulk and not at the surface, because

Fe(CN)64� anions are known to be repelled from the surface.137

The photoelectron spectrum obtained from such an experiment

is displayed in Fig. 11 (error bars are indicated). The strongly

rising photoelectron signal near 4.5 eV results from the direct

ionization of the ferrocyanide anion. The photoemission line

peaking at 3.3 eV with a linewidth of 0.8 eV is observed only in

the presence of the pump pulse. The photoelectron spectrum in

Fig. 11 is the first direct measurement of the binding energy of a

bulk hydrated electron in liquid water.

A number of experiments by other groups using liquid-jet

technology, but employing UV light (267–213 nm) instead of

EUV light for ionization, have been reported recently as well.

In these experiments, the hydrated electrons were generated

also by CTTS excitation of anions (Fe(CN)64� or I�). From

experiments of Tang et al.,130 a vertical electron binding

energy of 3.27 eV was concluded, whereas Shreve et al.131

and Lubcke and co-workers132 measured 3.6 eV and 3.4 eV,

respectively. Small deviations on the order of 0.1–0.2 eV in the

measured binding energies of the hydrated electrons may arise

from different anions as electron sources, different salt con-

centrations, or different (not perfect) calibration conditions or

beam charging. However, from all available experiments an

average reference value (3.4 � 0.2 eV) for the ionization

potential of the hydrated electron in liquid water can be

deduced. As noted above, an additional source of error may

arise from the calibration procedures used in ref. 130–132 which

are based upon gas-phase measurements rather than on liquid-

phase measurements. Shreve et al.,131 who generated hydrated

electrons from (I�)aq and (Fe(CN)64�)aq, could show that the

binding energy of (e�)aq is insensitive to the choice of the precursor.

The experiments by Tang et al.130 and Lubcke et al.132 additionally

revealed some new information on the solvation dynamics

following CTTS excitation of I� in aqueous solution and provided

a somewhat more complete picture of the ultrafast ejection,

solvation and recombination dynamics of the hydrated electron.

In an additional experiment by Siefermann et al., liquid

water in a micro-jet was excited with two 267 nm photons,

corresponding to an excitation energy of 9.30 eV, just below

the ionization potential of liquid water.29–31 The measured

EUV photoelectron emission spectrum is displayed in Fig. 12.

Contrary to the spectrum in Fig. 11, the photoemission peaks

at 1.6 eV binding energy (with a somewhat larger width of

1.3 eV) appear—as in the previous case—only in the presence

of the 267 nm pump pulse. This spectrum, which has been

generated by two-photon excitation of water, was assigned to

a novel type of hydrated electron. Surprisingly, identical

pump–probe photoelectron spectra were obtained when the

delay time of the EUV probe pulse was varied between 2 and

100 ps, which implies that the lifetime of this transient, which

was finally assigned to a surface-bound hydrated electron by

Siefermann et al., exceeds 100 ps.

A photoemission signal of the less strongly bound hydrated

electron (1.6 eV binding energy) could only be detected by UV

two-photon excitation of neat water and with EUV ionization.

The assignment of the transient to correspond to a surface-

bound electron is consistent with the very low probing depth

of EUV generated electrons.138,139 The differences to the

experiments not observing the surface-bound electron are the

precursor and the probe wavelength. With respect to the latter,

it is important to realize that the photoemitted electrons

generated by the EUV probe pulse possess kinetic energy of

about 35 eV, which is near the minimum of the electron

attenuation length (EAL) curve of about 2 mm,138,139 making

the ultrafast probe very surface selective for short-lived species.

In the laser photoionization experiments of Tang et al.,

Fig. 11 Photoelectron signal of the interior solvated electron in the

liquid jet.129Fig. 12 Photoelectron signal of the surface-bound electron in the

liquid jet.129

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Shreve et al., and Lubcke et al., on the other hand, the

photoelectron kinetic energies are of the order of 1 eV, for

which the EAL is at least one order of magnitude larger, which

explains why the surface-bound electron could not be detected,

although Lubcke et al. report an excellent dynamical range of

their set-up (c1000). Another reason may be the choice of the

different precursors, which may be an indication that this

surface species can be generated by two-photon excitation of

water, but not by laser photodetachment of solvated anions.

This conjecture is supported by the fact that another surface-

sensitive technique, second harmonic generation (SHG), also

failed to detect a surface-bound electron when applied to

hydrated electrons generated by CTTS excitation of I�

anions.140 The hydrated electrons prepared by anion photo-

detachment in salt solutions and by two-photon UV excitation

of neat water thus may be physically or chemically distinct

species. Recently, the Mafune group reported spectra of iodide

anions hydrated at the surface and in the bulk, respectively.

It was pointed out that excitation with different wavelengths

(and thus excitation of different CTTS bands) may produce

different transients of the hydrated electron.141

Shedding light on the energetics and qualitative binding

motifs of the bulk hydrated electron, (e�)aq, and the surface-

bound hydrated electron, (e�)sf, is quite important for the

understanding of how electrons can attach to molecules in

aqueous environments and break covalent bonds. A cartoon

picture of a typical bond-breaking situation for an organic

molecule is displayed in Fig. 13. On the left-hand side, the

binding energy (ruler) of the solvated electron and its photo-

emission spectrum are shown. As has been discussed in detail

in ref. 129, the energy acceptance window of organic molecules

with energetically suitable LUMOs (lowest unoccupied orbitals)

for solvated electrons is governed by the energies for vertical

electron attachment (VEA), adiabatic electron attachment

(AEA), as well as the vertical electron detachment energy (VDE)

of the target molecules. As can be seen in Fig. 13, the surface-

bound solvated electron is often approximately isoenergetic with

the electron attachment energy of many organic molecules, such

as DNA or CFCs (chlorinated and/or fluorinated hydrocarbons),

for example. Thus, the measured binding energies of solvated

electrons provide reference values for important electron-transfer

processes taking place in aqueous solution, in particular resonant

dissociative electron attachment (RDEA) processes. The VBEs of

(e�)aq and (e�)sf together with the RDEA model allow us to

understand how electrons in aqueous solution can easily break

strong covalent bonds in systems like DNA or CFCs. This

qualitative picture reveals (in a semiquantitative fashion) why

(e�)sf as well as optically excited hydrated electrons may play an

important role in radiation-induced DNA damage and organic

molecule bond cleavage. RDEA of organic molecules like CFCs

indicates that the (e�)sf transient may also be involved in atmo-

spheric ozone chemistry, although the main reaction channels are

dominated by radicals rather than ionic species.129

V. Synopsis: what do we know about the solvated

electron in clusters, at interfaces and in the bulk

The measured binding energies of the hydrated electron

prepared by laser photodetachment from anions in a liquid

jet are 3.3 eV,129,130 3.4 eV,132 and 3.6 eV,131 which lead to a

not very accurate, but reliable reference value (3.4 � 0.2 eV)

for the ionization potential of the hydrated electron in liquid

water. A more weakly bound electron (1.6 eV binding energy),

prepared by UV two-photon excitation of neat water, could be

detected only with EUV ionization.129 The interpretation as a

surface-bound hydrated electron is consistent with the very

low probing depth of EUV generated electrons.129 The

existence of a long-lived (>100 ps) surface-bound electron in

liquid water with an ionization potential of 1.6 � 0.1 eV has

thus been established for the first time. While surface-bound

hydrated electrons with low electron binding energies have

frequently been observed in cold anionic water clusters, this is

the first report of a surface-type electron in water at ambient

temperatures. The observation of two significantly different

ionization potentials of the solvated electron in liquid water is

a surprising and important finding in the long and entangled

history of the hydrated electron. The notion of a single

equilibrated hydrated electron species in liquid water, which

pervades all previous experimental and theoretical research in

the condensed phase, seems to be an oversimplification.

It should also be kept in mind that the two species

with B3.4 eV and 1.6 eV binding energies, respectively, were

prepared in a different manner. The hydrated electron with the

larger binding energy was generated by a CTTS transition in a

salt solution.129–132 In this case, the solvated excess electron is

accompanied by a positive counterion (Na+ or K+ in the

experiments discussed here). It is plausible that the positive

counter-ion stabilizes a rather localized electronic charge distribu-

tion in the interior of the solvent, which can explain the larger

binding energy of the bulk hydrated electron compared to the

surface-bound hydrated electron. Calculations predict that

the electronic absorption spectrum and the binding energy of

the solvated electron are essentially independent of the counter-

ion (Li+, Na+, K+, etc. in the case of salt solutions, or H3O+ in

the case of neat water or acidic solutions).27,142

The surface-bound electron in liquid water was generated by

excitation of neat water with two 267 nm photons.129 The

excitation energy of 9.30 eV is below the ionization potential

of liquid water.40 A water molecule is thus excited to a high

Rydberg state. If the excitation occurs sufficiently close to the

surface, the diffuse Rydberg orbital is ‘‘squeezed’’ out of the

Fig. 13 Schematic view of the energy acceptance window for solvated

electrons depending upon the VEA (vertical electron attachment energy),

the VDE (vertical detachment energy), and the AEA (adiabatic electron

attachment energy).129

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tight hydrogen-bond network of water, whereas the H2O+

cation is stabilized (at least for very short times) by solvation

within the liquid. The measured binding energy of the surface-

bound electron in water129 is consistent with the binding

energy of surface-type hydrated electrons in cold (H2O)n�

clusters.79–81 The bulk solvated and the surface-bound electrons

may thus be distinct (chemical) species (a (M+)aq (e�)aq contact

ion pair in the former case, and a separated (H2O+)aq (e�)aq

pair in the latter case). This concept can explain the long

lifetime of the surface-bound electron in liquid water. Other-

wise, the existence of a substantial barrier for the incorporation

of the surface electron into the bulk has to be assumed. Such a

barrier seems unlikely in warm, liquid water.82–84

With the knowledge of definitive values of the ionization

potentials of surface and interior excess electrons in liquid

water at room temperature, we may return to the analysis of

the extrapolation of the (H2O)n� and M(H2O)n cluster data

towards n = N (cf. Section III). The early data set65 for the

group I water anions extrapolates nicely to the newly established

ionization potential of the bulk hydrated electron, see Fig. 5. The

more recent data for larger clusters, on the other hand, appear to

extrapolate to an ionization potential of 3.6 eV74 or 4.0 eV,81

somewhat larger than the bulk value, see Fig. 5. This finding is

surprising in view of the lack of a positive counter-ion in these

clusters. Could it be that the excess electron induces ionic

dissociation of a water molecule in large clusters, forming

(H3O+)aq, (e�)aq and (OH�)aq? The less effectively screened

interaction of (H3O+)aq and (e�)aq in finite-size clusters could

explain the unexpectedly high value of the ionization potential of

(e�)aq in (H2O)n� clusters. The water cluster anions of group II

exhibit smaller electron binding energies which extrapolate nicely

to the newly determined value (1.6 eV) of the surface-bound

hydrated electron when only the cluster data for the larger sizes

are taken into account (see Fig. 5).74 This fact strongly supports

the assignment of the group II clusters as surface-bound anions.

This interpretation implies that the binding energy of the surface-

bound electron in the liquid jet is not affected too much by the

presence of the H2O+ counter-ion.

The deciphering of the electronic and molecular structures

of the multiple isomers of large water cluster anions may not

be possible without predictive theory. An important step

forward has recently been achieved by the calculation of

electron binding energies of large (H2O)n� clusters with

ab initio methods for all electrons.143,144 The results obtained

for (H2O)�104 are included in Fig. 5 as violet triangles. The

explicit values of the binding energies of (H2O)�104 are 2.05 eV for

the compact (interior) structure and 1.28 eV for the diffuse

(surface) structure.144 The agreement with the experimental

data is very satisfactory. Interestingly, the calculation predicts

also a compact surface state with a binding energy of 1.84 eV,

which does not fall into either group I or group II and which

has not been detected yet. According to these calculations, the

binding energy is less correlated with the location of the excess

electron on the surface or in the interior but with the compactness

of the electron density distribution. Large binding energies

correspond to compact and small binding energies to diffuse

electron distributions. This correlation has been suggested

previously by calculations for smaller cluster sizes of n = 32

as well.88 In addition, the same authors found that the process

of electron attachment to a neutral water system and subse-

quent localization is quite different for ambient and cryogenic

conditions.82,84 In the former system, the cluster quickly reaches

an equilibrated structure, which corresponds to a well localized

and strongly bound solvated electron. In contrast, in a cold

system the electron gets trapped in a metastable ‘‘cushion like’’

state at the periphery of the cluster, which is more weakly

bound. This rationalizes the observation of several isomers in

different cluster environments. It is plausible that anionic

clusters with a diffuse electron distribution extrapolate to the

weakly bound diffuse state of the liquid (surface state), while

the clusters with a compact electron distribution correlate with

the compact distribution in the liquid (interior state). We note

that despite of the fact that the clusters are solid-like and the

liquid is fluid, this extrapolation is justified, since the properties

of the solvated electron are dominated by the different shapes of

the electron density and not so much by the solid/liquid phase

of the aqueous environment.

A common feature of the solvated electrons prepared by

laser photodetachment of salt anions (X�) in bulk water and

by dissolution of alkali atoms (M) in finite-size water clusters

is the presence of a positive counter-ion. The neutral X�, which

is generated by the photodetachment of X�, interacts weakly

with the electron and the solvent and apparently does not

affect the electronic spectra and the electron binding energy. If

the escape length of the electron is large enough and if it does

not recombine via geminate recombination with the precursor,

we may regard it (and its binding energy) to be unperturbed by

the counter ion.

It is nevertheless interesting to compare the electron binding

energies of neutral M(H2O)n clusters with the electron binding

energy of the bulk solvated electron in the liquid jet. They

should only be equal if the electron in the former case is

sufficiently far away or unperturbed from the cation, i.e., in

sufficiently large water clusters. The extrapolation of the

ionization potential of the most prominent group I of

sodium–water clusters yields 3.2 eV (see Fig. 6), in good

agreement with the ionization potential of the hydrated

electron photodetached from Fe(CN)64� anions in the liquid

jet (3.3 eV). Simulations of the electronic structure of

M(H2O)n clusters indicate that the electron distribution is

localized and corresponds to an electron–ion contact pair

configuration.111 The ionization potentials of the group II of

Na(H2O)n clusters extrapolate towards 2.8 eV, see Fig. 6. A

corresponding species in the bulk has not been detected so far.

A comprehensive review of the properties of the solvated

electron in other solvents than water is beyond the scope of

this perspective. However, a brief comparison of the extra-

polation of the aqueous cluster ionization potential towards

the bulk limit with the corresponding data for two other

representative solvents, ammonia and methanol, is illuminating.

The data available for (NH3)n� clusters and Na(NH3)n clusters

are shown in Fig. 14.145 The ionization potentials of Na(NH3)nclusters decrease approximately linearly with n�1/3, while the

electron detachment energies of the (NH3)n� anions increase

linearly with n�1/3. We note that the curves in these plots

reproduce the measured points very well. They are calculated

based on the continuum theory. Here it is crucial that the static

and the optical dielectric constants of the solid material are

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used, in accordance with the probable thermodynamic state of

the clusters under the specific production conditions. How-

ever, neither extrapolation seems to lead to the ionization

potential of the solvated electron in bulk ammonia, see

Fig. 14.146,147 The deviations can be explained by the presence

of the counter-ion in Na(NH3)n clusters or the distance

between them and the solvated electron which is larger in

the liquid than in the solid clusters.

The results for methanol are similar, with the exception that

the ionization potentials of Na-doped clusters are constant for

large n. The ionization potential of the solvated electron in

liquid methanol has very recently been determined in a liquid

jet by laser photoionization of a NaI solution.148 Data for

Na(CH3OH)n clusters149 and (CH3OH)n

� clusters150 also have

been reported recently. The scaling of the ionization potentials

of the clusters with n�1/3 and the value of the bulk ionization

potential (3.1 � 0.1 eV) are displayed in Fig. 14. Two groups

(I, II) of cluster anions with large and small electron detach-

ment energies, respectively, have been observed. In both cases,

the electron detachment energies scale linearly with n�1/3, see

Fig. 14. As in the case of ammonia, the measured points are

reproduced by calculations with the exception of the constant

part of the ionization potentials of Na(CH3OH)n clusters

which show a behavior which is reminiscent of water. After

an initial decrease of up to n = 6, the ionization potential

remains constant and extrapolates towards 3.2 eV, in good

agreement with the measured bulk value, see Fig. 14. The

electron detachment energies of the anions, on the other hand,

extrapolate to electron binding energies which are significantly

lower than the binding energy of the bulk solvated electron. As

observed in ammonia, a stabilizing interaction with the Na+

counter-ion exists in the limit n - N. The solvated electrons

with and without positive counter-ions are thus different

species in methanol as well as in ammonia. We note that

problems with the extrapolation of excess electron properties

of amorphous solid clusters to the liquid bulk have been

predicted earlier.82,84

In the comparison of the three solvents water, ammonia and

methanol illustrated in Fig. 5, 6 and 14, the data for water are

clearly exceptional and special. The ionization potentials of

the group-I cluster anions extrapolate towards the ionization

potential of the bulk solvated electron. The ionization poten-

tials of the group-I Na(H2O)n clusters seem to extrapolate to

the same value within the accuracy of the measurements, see

Fig. 6. The ionization potential of an excess electron in large

water clusters is thus not affected by the presence of a positive

counter-ion, in contrast to ammonia and methanol clusters.

The surface-bound electron in large (H2O)n� clusters extra-

polates linearly in n�1/3 towards the surface-bound Rydberg

state observed in the liquid jet. Again, the electron binding

energy is not affected by the presence of a H2O+ counter ion in

the case of neat water. Ammonia, methanol, and water are all

hydrogen bonded systems. For water and methanol the bind-

ing energy of the hydrogen bond is significantly larger than for

ammonia. The structure of water is dominated by a four-fold

coordination, while methanol is arranged in linear long chains.

This leads to strong short-ranged interactions in water relative

to which counterions play a minor role. Another exceptional

feature of water is its incipient ionic dissociation in very large

clusters and in the bulk. As a result, excess electrons may

induce H3O+ counter-ions in neat water, an effect which blurs

the distinction between clusters with and without positive

counter-ions.

An open question which has found relatively little attention

in the literature is the origin of the finite lifetime of the

equilibrated solvated electron, which is believed to be of the

order of microseconds in neat liquid water.1 In photoexcita-

tion of HX(H2O)n clusters, clear evidence for the formation of

metastable H3O(H2O)n clusters has been found, but the

lifetime of these species was too short to be measured in these

experiments.117,122,127 By extension of the time delay in liquid-

jet photoionization measurements beyond nanoseconds, it

may be possible to measure the lifetime of the hydrated

electron in the electronic ground state under well-controlled

conditions.

VI. Conclusions and outlook

We have surveyed the currently available knowledge on the

binding energies of solvated electrons in various size-selected

aqueous clusters as well as in the liquid water jet. In liquid

water, the existence of two solvated electron species with

binding energies ofB3.4 eV andB1.6 eV has been established

so far. The species with 3.4 eV binding energy is tentatively

identified as the equilibrated solvated electron in water under

ambient conditions with a compact electron distribution. This

assignment is tentative insofar as the optical absorption

spectrum of this species, which is the defining property of

the hydrated electron, has not yet been measured. The species

with the lower binding energy (1.6 eV), which could be

detected only with EUV ionization and with two-photon

excitation of water as a precursor, has tentatively been

assigned as a surface-bound hydrated electron in liquid water.

The lower binding energy indicates a more extended electron-

density distribution of this species. Since the interior electron

and the surface-bound electron have been prepared in different

Fig. 14 Ionization potentials and vertical electron detachment

energies for ammonia145 and methanol149,150 as a function of the cluster

size. Blue points: ionization potentials of neutral Na-doped clusters.

Red points: detachment energies of negatively charged clusters. Green

points: measured electron binding energies of the liquid.146–148

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manners (by CTTS excitation of a salt solution and by

two-photon UV excitation of neat water, respectively), they

could be chemically distinct species. The interior electron

prepared in salt solutions may be associated with an alkali

counter-ion, while the equilibrated surface-bound electron

prepared in neat water presumably is separated from its

original H2O+ counter-ion.

We have summarized and discussed the large amount of

electron binding-energy data which are available for (H2O)n�

and Na(H2O)n clusters of various sizes and temperatures. In

the plot of these electron binding energies vs. n�1/3, certain

groups of isomers can readily be identified. The binding

energies of two of these groups seem to extrapolate towards

the two binding energies measured in the liquid jet. The data

are consistent insofar as the binding energies of a group of

clusters with an internally solvated electron mainly extrapolate

towards the binding energy of the internally solvated electron

in the liquid (3.4 eV), while the binding energies of a group of

clusters carrying a relatively weakly bound surface-type

electron extrapolate towards the binding energy of the surface-

bound electron in the liquid (1.6 eV). In addition, they also agree

in the type of the electron distribution, that is, compact for the

internally bound electron and dispersed for the surface bound

electron. We note, however, that there exist isomers of (H2O)n�

as well as of Na(H2O)n clusters which do not have an analogue in

the liquid, e.g., a group of (H2O)n� clusters with rather large

binding energies, which seem to extrapolate towards 4.0 eV, or a

group of Na(H2O)n clusters with a size-independent binding

energy of B2.8 eV (see Fig. 5 and 6).

The comparison of the electron binding energies of anionic

or alkali-doped clusters of water, ammonia and methanol

clearly reveals the ‘‘anomalous’’ solvation properties of water.

For ammonia and methanol, the extrapolated binding energies

of clusters carrying an excess electron are significantly lower

than the corresponding binding energies of alkali-doped

clusters, revealing the stabilizing effect of the positive counter-

ion in the alkali-doped clusters, see Fig. 14. With the exception

of alkali-doped methanol clusters, the extrapolated binding

energies of these clusters differ from the corresponding binding

energy of the liquid. In the case of water, on the other hand,

there exist clusters with internally bound electrons as well as

with surface-bound electrons with ionization potentials which

extrapolate towards the corresponding ionization potentials of

the liquid, see Fig. 5 and 6. The electron binding energies of

aqueous clusters seem to be determined by specific and

relatively short-ranged hydrogen bonding interactions, rather

than long-range dielectric screening effects. An alternative

explanation of the anomalous solvation phenomena in large

water clusters as well as in liquid water could be the unique

tendency of water towards ionic dissociation. An excess electron

in water may thus generate its own H3O+ counter-ion, resulting

in an overall neutral (H3O+)aq (e

�)aq solvated ion pair and an

(OH�)aq anion.

The experiments on the photodissociation of hydrogen-

halide doped water clusters, HX(H2O)n, have provided strong

evidence of the existence of the hydronium radical, H3O, in the

radiation chemistry of aqueous clusters. The HXmolecules are

acidically dissociated at the surface of water clusters (as well as

in the liquid). When photoexcited to a CTTS state, they relax

to an XH3O(H2O)n�1 biradical. Both the H3O radical as well

as the X radical have definitively been detected in cluster

experiments, the former through its decay to H2O + H and

the characteristic kinetic-energy distribution of the released

H atoms. For the liquid phase, such a direct proof of the

relevance of the H3O radical is lacking. Calculations indicate,

however, that the H3O radical is effectively solvated as a

(H3O+)aq (e�)aq ion pair and that this ion pair carries the

characteristic spectroscopic properties of the bulk hydrated

electron.25,28 The hydronium cation, H3O+, may thus be the

counter-ion of the bulk hydrated electron under certain

circumstances.

The recent successful preparation and time-resolved

detection of the hydrated electron in liquid water jets via photo-

emission spectroscopy—especially with extreme UV radiation—

opens up new opportunities and avenues for the investigation of

the structure and the dynamics of this elusive species, in particular

at the interface of water due to the surface selectivity of EUV

photoelectron spectroscopy. Nevertheless, even more than

50 years after the discovery of the hydrated electron, its dynamics,

binding motifs and energetics are still not as well known as is

desirable for the simplest of all chemical species in aqueous

solution. Fundamental questions which remain to be answered

are: (i) Can the interior hydrated electron be generated in the

liquid jet by two-photon UV excitation of neat water? (ii) What

happens when the excitation/generation wavelength is tuned over

an extended range? (iii) Can we obtain insight into RDEA

processes with hydrated electrons as intermediates with time-

resolved experiments?

As demonstrated in ref. 129–132, the relaxation dynamics of

the nascent hydrated electron towards the equilibrated

hydrated electron can now be followed by time-resolved

photoelectron spectroscopy, which certainly will provide novel

information on the structure and relaxation kinetics of the

transient species in electron solvation (the incompletely

solvated or ‘‘wet electrons’’48,49). Polarization-resolved photo-

emission spectroscopy is expected to emerge soon and will

have the potential to provide novel information on the

electron density distribution at the interface and in the bulk.

Work along these lines is presently underway in our laboratories.

Clearly, theory is needed to help us in the development of the

complete picture of the structure and dynamics of the hydrated

electron.

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