Computational Results
Supplemental Figure 1: Correlation between the strength of donating (blue) and accepting
(red) hydrogen bonds of a water molecule and the Mulliken spin population on its oxygen
atom. The strength is quantified simply by the distance between the oxygen and hydrogen
atom forming the hydrogen bond. The dashed line denotes a spin population cutoff of 0.05
below which water molecules are not considered. The shaded regions encompassing the
dots are meant to guide the eye.
What features characterize the water molecules on which the initial spin density resides? In
order to address this question we have analyzed the initial (i.e., neutral) geometries of each
trajectory in terms of electronic structure and geometric properties of individual water molecules.
1
A strong correlation is found with the value of the molecular Highest Occupied Molecular
Orbital (HOMO) as computed using the procedure described by Hunt and Sprik.1 That is, the
initial spin density tends to reside on a molecule for which the value of the HOMO eigenvalue is
near the top of the band of occupied orbitals, which is consistent with the simple picture that the
electron is removed from a high lying orbital. The position of the molecular HOMO is influenced
by several factors, such as the molecular geometry, the local electrostatic potential, and the
hydrogen bonded network around the individual molecules.2
Supplemental Figure 1 shows the correlation between the amount of initial spin density on a
water molecule and the “quality” of hydrogen bonds to its neighbors, which we simply relate to
the corresponding O...H hydrogen bond distance. Prolonging the O...H distance in a bond where
the investigated water molecule acts as a hydrogen bond acceptor should lead to destabilization
of the corresponding lone pair orbital, making it a more likely candidate for initial spin density
after ionization. On the other hand, shortening the O...H distance in a bond where the
investigated water molecule acts as a hydrogen bond donor moves the ionized system along the
reaction path, should the initial spin density be appreciable on that water molecule. Separately,
we thus monitor donating and accepting hydrogen bonds of each of the investigated water
molecules. Blue dots in Figure 6 represent the shorter of the two donating and red dots the longer
of the two accepting O...H hydrogen bonds. While we do not see any strong relation between the
“quality” of the donating bonds and initial spin population, there is a correlation for the accepting
bonds. Namely, it follows from our analysis that the initial spin density tends to be positioned on
water molecules which have weakened accepting hydrogen bonds to their neighbors (i.e., the red
region in Figure 6 is tilted to the right). Such situations lead to destabilization of the lone pairs on
these oxygens, which then become the orbitals from which (lowest) ionization takes place.
2
Previously studied small water clusters, where the cationic hole is spread over one or several
water molecules with no accepting hydrogen bonds,3 represent an extreme demonstration of this
behavior.
Supplemental Figure 2: Maximum Mulliken spin population on a single oxygen atom, shown
as a function of time for different electronic structure methods. Spin populations greater than 1.0
are due to the use of unrestricted Hartree-Fock and hybrid calculation. The selected trajectory is
the same as in Figure 6 of the main paper.
The initial degree of spin delocalization can be expected to correlate with the timescales of the
spin localization processes. Supplemental Figure 2 shows the time evolutions of the largest
3
Mulliken spin population found on single water molecule oxygen, as obtained for trajectories
started from identical initial conditions as presented in Figure 6 of the main paper. We see for
different methods that the spin localization process is indeed directly connected with the initial
delocalization. In one extreme case, the initially over-delocalized BLYP spin density does not
localize at all, at least not on the timescale of the present simulation. In the other extreme case,
the initially completely localized Hartree-Fock cationic hole remains localized on a single center
for the whole trajectory. Note that due to the use of the unrestricted formalism of the HartreeFock theory the spin population presented in Figure 8 can even exceed unity. Neither of these
results is correct and they merely provide the two extreme situations. As also shown above, the
presently employed BLYP-SIC approach leads to a rapid localization (within about 15 fs for the
trajectory shown in Figure 7) of the initially partially delocalized cationic hole. The two HSE
calculations with 25 and 50 % of exact exchange behave similarly with the speed of localization
being proportional to the amount of exact exchange mixed in. Note that again the BLYP-SIC
result lies between the two HSE ones.
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Experimental Results
Supplemental Figure 3: Contour plots of experimental transient absorption for two-photon
ionization of liquid water with parallel (top panels) and perpendicular (bottom panels)
relative polarization. Excitation wavelength is 227 nm pump, probe is white-light
continuum obtained by focusing 800 nm laser pulses into a 3 mm CaF2 plate. The narrow
contour range in the left panels saturates (white areas) both the two-photon absorption
(2PA) at time-zero and the transient solvated electron signal for wavelengths greater than
~450 nm. The negative signal (red) immediately following the 2PA band is due to crossphase modulation (XPM). 2PA and XPM signals are roughly 3 times stronger for parallel
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than for perpendicular polarization. The panels on the right show that the solvated electron
signal increases with a timescale of ~1-2 ps, giving a transient absorption at the 720 nm
band maximum (not shown) of approximately 20 mOD.
Supplemental Figure 4: (Top panel) Spectral cuts of the transient absorption after twophoton ionization of water, 2 x 227nm, perpendicular polarization from Figure S1
compared against the expected contribution to the signal from OH radicals. The absorption
increases to longer probe wavelengths due to the contribution from the solvated electron
even at short delays, whereas the absorption at shorter wavelengths includes contributions
from solvated electron and OH.
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(Black) Representation of data for the aqueous OH radical spectrum at 350OC
reported in ref. 46 where few hydroxyl radicals have established hydrogen bonded
geometries with neighboring waters. An absolute extinction value is reported in ref. 46
allowing us to estimate the signal level of OH relative to solvated electrons (see main text)
but the original spectra are reported only to λ ≤ 320 nm. We have thus been forced to
Gaussian fit the original data and extrapolate the spectra to longer wavelengths which may
arbitrarily cut off the red wing of the spectrum.
(Green; Bottom Panel in detail) An alternative way to assess the experimental OH
spectral shape over the 300 – 400 nm range comes from independent experiments where
OH is produced by one-photon photodissociation of aqueous H2O2 at 227 nm (ref. 60). The
spectrum shown is the transient absorption at ~ 100 fs delay for “hot” OH emerging from
the photodissociation reaction, scaled in top panel to the same peak absorbance as the
black curve.
Considering the estimated decomposition of the water ionization signal, it can be
seen that the OH is difficult to cleanly discern from the electron signal making it hard to
pick up a clear signature of time-evolving signal from H2O+; in the current experimental
realization, we conclude we do not observe such a spectral dynamic (see text).
References:
1
2
3
P. Hunt and M. Sprik, Chemphyschem 6 (9), 1805 (2005).
M. J. McGrath, J. I. Siepmann, I. F. W. Kuo, C. J. Mundy, J. VandeVondele, J. Hutter, F.
Mohamed, and M. Krack, Journal of Physical Chemistry A 110 (2), 640 (2006).
P. A. Pieniazek, E. J. Sundstrom, S. E. Bradforth, and A. I. Krylov, Journal of Physical
Chemistry A 113 (16), 4423 (2009).
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