Information Content in O[1s] K-edge X-ray Emission Spectroscopy of Liquid Water :

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Information Content in O[1s] K-edge X-ray Emission Spectroscopy of Liquid Water
Michael Odelius*
FYSIKUM, Stockholm UniVersity, AlbanoVa, S-106 91 Stockholm, Sweden
ReceiVed: April 3, 2009; ReVised Manuscript ReceiVed: June 2, 2009
Does the fine-structure in oxygen K-edge X-ray emission (Fuchs et al. Phys. ReV. Lett. 2008, 100, 027801)
imply that liquid water is a two-component mixture or is it the signature of a transient OH species arising in
the core-excitation process? As with the interpretation of the X-ray absorption spectrum of liquid water, this
question is also intensely discussed in the water and X-ray spectroscopy communities. X-ray emission is an
independent probe of the electronic structure yielding complementary information on hydrogen bonding in
liquid water. In this study, the angular anisotropy in the resonant inelastic soft X-ray scattering (resonant
X-ray emission (XE)) spectrum of liquid water is simulated on the basis of ab initio molecular dynamics
simulations to allow for direct comparison to recent experimental data (Forsberg et al. Phys. ReV. B 2009, 79,
132203). Theoretical simulations unequivocally show that the difference in angular anisotropy in the water
lone-pair features is related to their fundamentally different origin. The high emission-energy peak is primarily
due to the contribution from the out-of-plane (1b1) lone-pair in intact water molecules. On the other hand, the
low emission-energy lone-pair peak originates from the bonding (3a1) state and is assigned to a transient OH
species formed by ultrafast (<10 fs) photodissociation. The information in the XE spectrum on the structure
of liquid water is limited and buried in features arising from excited state dynamics. In combination with
available experimental data, the theoretical simulations settle a rising debate on the interpretation of resonant
and nonresonant XE spectra of liquid water and there are strong implications for the XE spectroscopy of
hydrogen-bonded liquids. The simulations show that the fine-structure in the XE spectrum of liquid water
can be explained simply in terms of present day ab initio molecular dynamics simulations.
1. Introduction
X-ray emission (XE) spectroscopy1 is a unique tool for
obtaining element-specific electronic information in complex
systems. It is a valuable complement to other experimental
probes of the electronic structure in the condensed phase. After
core-excitation of a specific element, in this case oxygen, decay
of the O[1s] core-hole gives rise to emission of a X-ray photon
when a valence electron falls down to fill the core-hole. Thus,
the XE process can be viewed as a local projection of the
valence electronic structure depicting the local density of states
on the core-excited atom. Resonant X-ray emission, or more
accurately resonant inelastic soft X-ray scattering (RIXS),1
contains additional information. In RIXS, each region of the
X-ray absorption spectrum, and its associated intermediate coreexcited
states, results in a specific projection of the local density
of states. Hence, RIXS contains information not only on the
configurational selectivity of the core-excitation but also
processes occurring in the intermediate state as well as the
overlap between the initial, final, and intermediate electronic
states.
The specific aim of the present study is to provide a detailed
molecular interpretation of the recently published angular
anisotropy in RIXS at the oxygen K-edge (O[1s]) of liquid
water.2 The experimental data could be improved both in terms
of signal-to-noise ratio and the extension of the excitation
energies probed, but the data from the existing experiments are
already very informative. I show that extended studies of angular
anisotropy in RIXS could be a crucial piece of evidence in
understanding X-ray emission in liquid water.3-11 Furthermore,
a RIXS map over different excitation energies would provide
unique information on the character of the X-ray absorption
(XA) spectrum. The controversial claims about the hydrogen
(H-)bonding in liquid water from the interpretation of X-ray
absorption data12,13 were partly based on theoretical calculations.
Alte
ative theoretical methods have been used to reinterpret
the experimental data14,15 but there is no consensus on the
applicability of these methods.13-19 Additional electronic information
on the XA spectrum of liquid water is essential to
evaluate the approximations employed in the models used to
simulate XA spectra.
The recent experimental development of liquid cells and
liquid jets has enabled studies of XE and RIXS in the liquid
phase and in particular on water and other H-bonded liquids.
3-9,20,21 This method is relatively new in the field of
molecular liquids and so it is desirable to clarify its information
content and to determine how to extract useful information from
the data. In the literature, two incompatible classes of interpretations
of X-ray emission spectra of H-bonded liquids are
advocated.
On one hand, the fine-structure in the XE spectrum has been
employed to unravel amazingly detailed information on specific
molecular structures in liquid water, alcohols, and alcohol-water
mixtures.3,4,9,10,20,21 For liquid water at least, these conclusions
are at odds with state-of-the-art molecular dynamics (MD)
simulations.9