A time constant of 1.8 fs in the dissociation of water excited at 162 nm

Max-Planck-Institut für Quantenoptik, Hans-Kopfermann Straße 1, D-85748 Garching, Germany
Chemical Physics Letters (Impact Factor: 2.15). 01/2009; 468(1):9-13. DOI: 10.1016/j.cplett.2008.11.093

ABSTRACT Probing the first excited-state of H2O, HDO and D2O by ionization at 810nm reveals in the parent-ion yields time constants of 1.8, 2.1 and 2.5fs, respectively, during which the molecule leaves the Franck–Condon region, stretching the bonds of by about 0.25Å. The OH+ signal rises slightly more slowly (1.8+1.7fs), because only then is the dissociation energy of the parent ion overcome. The subsequent decay (3.3fs) is caused by the decreasing ionization probability. The detection of such short times is intimately connected with the sensitivity of the probe technique to geometrical changes in the sub-Ångström range.

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    ABSTRACT: The ionization of liquid water functions as the principal trigger for a myriad of phenomena that are relevant to radiation chemistry and biology. The earliest events that follow the ionization of water, however, remain relatively unknown. Here, femtosecond coherence spectroscopy is combined with polarization anisotropy measurements to elucidate the ultrafast electron and ion dynamics in ionized water. The results show that strong-field ionization of liquid water produces an aligned p electron distribution. Furthermore, oscillations observed in the polarization anisotropy are suggestive of valence electron motion in the highly reactive H 2 O + radical cation, whose lifetime with respect to proton transfer is found to be 196 ± 5 fs. Coherent intermolecular motions that signal initial solvent reorganization and subsequent long-lived ballistic proton transport that involves the H 3 O + end product are also detected in the time domain. These results offer new insight into the elementary dynamics of ionized liquid water. SECTION: Liquids; Chemical and Dynamical Processes in Solution T he ionization of liquid water is a universal phenomenon that accompanies the interaction of high-energy radiation with matter in aqueous environments. The ensuing cascade of chemical reactions that involves ions, electrons, and radicals forms the basis of solution and interfacial radiation chemistry. 1 Moreover, because liquid water comprises the major component of cellular matter, oxidative damage to biological systems by high-energy radiation is triggered mainly by the ionization of water. 2 The low-energy electrons that are produced by ionization can also induce radiation damage by dissociative electron attachment to biomolecules. 3 The elementary processes that follow the ionization of liquid water therefore form the fundamental framework for studies of radiation−matter interaction in chemistry and biology. The direct products of water ionization are the H 2 O + radical cation and the hydrated electron, 1 both of which are highly reactive species. Ab initio molecular dynamics (AIMD) simulations 4,5 and infrared spectroscopy of ionized water clusters 6 show that H 2 O + undergoes an ion−molecule reaction with a neighboring H 2 O molecule to yield the hydronium cation (H 3 O +) and a hydroxyl radical (OH). The H 3 O + product subsequently undergoes rapid structural diffusion, akin to the case of an excess proton in water, 7 thereby resulting in the rapid disappearance of the initial H 3 O +