Observation of the fastest chemical processes in the
radiolysis of water
Z.-H. Loh,
1∗
G. Doumy,
2
C. Arnold,
3,4,5
L. Kjellsson,
6,7
S.H. Southworth,
2
A. Al Haddad,
2
Y. Kumagai,
2
M.-F. Tu,
2
P.J. Ho,
2
, A.M. March,
2
R.D. Schaller,
8,9
M.S. Bin Mohd Yusof,
1
T. Debnath,
1
M. Simon,
10
R. Welsch,
3,5
L. Inhester,
3
K. Khalili,
11
K. Nanda,
12
A.I. Krylov,
12
S. Moeller,
13
G. Coslovich,
13
J. Koralek,
13
M.P. Minitti,
13
W.F. Schlotter,
13
J.-E. Rubensson,
6
, R. Santra,
3,4,5∗
L. Young
2,14∗
1
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences,
Nanyang Technological University, Singapore
2
Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL USA
3
Center for Free-Electron Laser Science, DESY, Hamburg, Germany
4
Department of Physics, Universit
¨
at Hamburg, Hamburg, Germany
5
The Hamburg Centre for Ultrafast Imaging, Hamburg, Germany
6
Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden
7
European XFEL GmbH, Schenefeld, Germany
8
Center for Nanoscale Materials, Argonne National Laboratory, Lemont, IL, USA
9
Department of Chemistry, Northwestern University, Evanston, IL, USA
10
Sorbonne Universit
´
e and CNRS, Laboratoire de
Chemie Physique-Mati
`
ere et Rayonnement, LCPMR, F-750005, Paris, France
11
Department of Energy Conversion and Storage,
Technical University of Denmark, Roskilde, Denmark
12
Department of Chemistry, University of Southern California, Los Angeles, CA, USA
13
LCLS, SLAC National Accelerator Laboratory, Menlo Park, CA, USA
14
Department of Physics and James Franck Institute, The University of Chicago, Chicago, IL, USA
∗
To whom correspondence should be addressed;
E-mail: zhiheng@ntu.edu.sg, robin.santra@cfel.de, young@anl.gov
1
One sentence summary: Ultrafast x-rays capture the elementary steps of proton transfer and
hydroxyl radical solvation in ionized liquid water.
Elementary processes associated with ionization of liquid water provide a frame-
work for understanding radiation-matter interactions in chemistry and biol-
ogy. While numerous studies have been conducted on the dynamics of the
hydrated electron, its partner arising from ionization of liquid water, H
2
O
+
,
remains elusive. We use tunable femtosecond soft x-ray pulses from an x-ray
free electron laser to reveal the dynamics of the valence hole created by strong-
field ionization and to track the primary proton transfer reaction giving rise to
the formation of OH. The isolated resonance associated with the valence hole
(H
2
O
+
/OH) enables straightforward detection. QM/MM calculations reveal
that the x-ray spectra are sensitive to structural dynamics at the ionization
site. We find signatures of hydrated-electron dynamics in the x-ray spectrum.
Radiolysis of liquid water is a universal phenomenon that accompanies the interaction of
high-energy radiation with matter in aqueous environments. It is of fundamental importance in
many domains (1), ranging from water-cooled nuclear reactors where radiolysis products cause
corrosion (2) to radiation-induced genomic damage in living organisms where water comprises
80% by weight (3), thereby making radiolysis foundational to medical treatment, diagnosis and
even extended human space flight (4). While ionizing radiation is delivered via various vehicles
(x-rays, γ-rays, charged particles) its interaction with matter can be understood conceptually as
individual absorption events along the particle path accompanied by a cascade of electrons, ions
and radicals (5).
Consider the most elementary process: ionization of pure liquid water which leads to the
2
formation of a hydrated electron precursor and a cationic hole (H
2
O
+
). Both species are very
reactive. The dynamics of the hydrated electron, e
−
aq
, has been the subject of numerous studies
(6–11) since the discovery of its visible spectrum (12), which peaks at 718 nm and spans 500-
1000 nm - convenient for ultrafast laser spectroscopies. In stark contrast, its ionization partner,
H
2
O
+
, has not been experimentally detected. The H
2
O
+
is predicted to undergo rapid sub-
100-fs proton transfer to a neighboring water molecule to yield the hydronium cation (H
3
O
+
)
and the hydroxyl radical (OH) (1, 14, 15). Attempts to observe directly the H
2
O
+
cation using
ultrafast visible or ultraviolet probes have been inconclusive due to its ultrashort lifetime and
masked spectral signature (6, 13, 15). Thus, basic questions regarding the ionization of water
remain. What is the lifetime of H
2
O
+
? What are the absorption spectra of H
2
O
+
and OH? What
is the extent of hole delocalization in H
2
O
+
and its timescale for localization relative to proton
transfer?
Here we introduce an ultrafast x-ray probe which enables us to track the primary chemical
reaction following ionization of liquid water, namely H
2
O
+
+ H
2
O → OH + H
3
O
+
. Combined
experiment and theory yield insight for the effort in this new spectral regime. X-rays are well-
suited for probing the short-lived H
2
O
+
cation and OH radical as their absorption lines fall
cleanly in the water window. Removal of an electron from the outermost valence orbital (1b
1
)
of H
2
O produces a new transition for H
2
O
+
that is red-shifted from the 1a
1
→ 4a
1
pre-edge
transition at 535 eV (16) roughly by the HOMO-LUMO gap ∆E (17), as shown in Fig. 1A.
The OH radical, isoelectronic to H
2
O
+
, possesses a nearby x-ray absorption resonance, while
the other product of proton transfer, H
3
O
+
, has resonances that fall in a region of strong water
absorption (Fig. 1B). The experimental configuration, consisting of a 60-fs, 800-nm strong-field
ionization pump (10), tunable ∼30 fs ultrafast x-ray probe from the LCLS (Linac Coherent
Light Source) x-ray free-electron laser (XFEL) (18) and three photon-in/photon-out detection
channels, is shown in Fig. 1C.
3
Signatures of the impulsively produced valence hole and excess electron appeared in all
three detection channels when the incident x-ray energy was scanned. Fig. 2A displays data
from the dispersed fluorescence channel, whereas the transmission mode is used in Figs. 2B,
C and D. Fig. 2A shows absorption prior to (∆t < 0 ps) and after ionization (∆t > 100 fs).
At negative time delay, the absorption is that of liquid water, i.e. nonresonant ionization of
the valence and inner-valence levels of water plus the pre-edge transition. (Saturation effects
prevented a clear observation of the pre-edge resonance.) At positive time delay, two new
features are apparent: an absorption resonance at 525.9 eV and a shift of the pre-edge absorption
to lower energies. The new absorption resonance is consistent with the creation of a hole in the
outermost valence level of liquid water. The corresponding H
2
O
+
transition energy can be
estimated to be 526.9 eV using the binding energies in liquid water of the O 1s core level
(538.1 eV) (19) and the 1b
1
HOMO (11.2 eV) (20). The isoelectronic OH exhibits a gas-phase
absorption peak at 525.85 eV (21).
The H
2
O
+
cation produced by the ionization of liquid water is widely expected to decay via
a pseudo-first-order reaction involving proton transfer to a neighboring water molecule
H
2
O
+
+ H
2
O → OH + H
3
O
+
(1)
to yield the OH radical and the H
3
O
+
species. In the absence of electron scavengers, the OH
radical subsequently undergoes geminate recombination (22) with the ejected electron to give
OH
−
. The time-resolved differential absorption spectrum, ∆A = A(∆t) − A(∆t < 0), showed
a prompt increase at time-zero, followed by a narrowing of the spectral width within the first
picosecond, and finally a gradual decay at longer time delays (Fig. 2B). We modeled this be-
havior as sequential kinetics: the species initially produced by ionization decays with lifetime
τ
1
to give an intermediate species with lifetime τ
2
, with absorption spectra S
1
(E) and S
2
(E),
respectively.
4
We assign S
2
(E), obtained by averaging ∆A for time delays between 1.5−5.8 ps, to the OH
radical (Fig. 2C). S
2
(E) can be fit to a sum of two Lorentzians, a main peak at 525.97 ± 0.08
eV and a ∼ 7× weaker sideband at 526.45 ± 0.12 eV. The 0.48-eV energy spacing between the
two peaks is in reasonable agreement with the 0.53-eV spacing of the vibrational progression
for core-excited gas-phase OH (21). Both peaks have a common FWHM (full width at half
maximum) of 0.48 ± 0.02 eV, significantly broader than the 0.1-eV spectral bandwidth of the
XFEL pulses and the 0.147-eV natural linewidth (21), suggesting inhomogeneous broadening
by the solvent environment.
With the x-ray absorption lineshape of the aqueous OH radical S
2
(E) determined, the next
step is to extract the time constants τ
1
and τ
2
, and the spectral lineshape of the short-lived com-
ponent S
1
(E). We used a Gaussian instrument response function of 106-fs FWHM and per-
formed a surface fit of the experimental data shown in Fig. 2B. (We did not convolve the kinetic
model with the experimental energy resolution of 0.1-eV FWHM because the experimentally
observed spectral features were much broader.) The surface fit yielded τ
1
= 0.18 ± 0.02 ps
and τ
2
= 14.2 ± 0.4 ps and a Lorentzian absorption lineshape S
1
(E) centered at 526.01 ± 0.13
eV with a FWHM of 0.98 ± 0.04 eV. The decay of the OH radical, τ
2
, most likely originates
from geminate recombination of the OH radical with a hydrated electron. The recombination
time constant of 14.2 ± 0.4 ps is significantly shorter than those reported in the literature (22),
most likely due to the high initial ionization fraction accelerating the geminate recombination
process. Geminate recombination between e
−
aq
and OH in ionized liquid water has been ex-
tensively investigated by time-resolved optical spectroscopy (22) and is beyond the scope of
the present study. Nevertheless, considering the relative diffusion coefficients of the reactants
and the reaction radius, our observed timescale for geminate recombination suggests an ap-
proximate ionization fraction of 1.7%, in relatively good agreement with the ionization fraction
estimated based on the OH x-ray absorbance (see below). The spectral lineshape S
1
(E) and
5