A time constant of 1.8 fs in the dissociation of water excited at 162 nm
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 H2O+ 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 H3O+ end product are also detected in the time domain. These results offer new insight into the elementary dynamics of ionized liquid water.
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ABSTRACT: In the frame of the time-dependent local-density approximation coupled nonadiabatically to molecular dynamics (MD), we explored the optical absorption strength and dynamics of a water dimer in intense laser pulses with different intensities. The optical absorption spectra of a water dimer exhibiting well marked and isolated peaks agrees well with the results obtained by ab initio MD simulations. Three typical possible reaction paths are exhibited, which are normal oscillation, OH bond break and Coulomb explosion, respectively. The ionic motion shows that the water donor and the bonded OH bond are more sensitive to the laser intensity. Moreover, the water donor determines the structural reorganization of the water dimer. It is also found that the larger intensity causes higher ionization and brings forward H-transfer.Laser Physics 08/2014; 24(10):106004. DOI:10.1088/1054-660X/24/10/106004 · 1.03 Impact Factor
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ABSTRACT: Experimental estimates of photolytic efficiency (yield per photon) for photodissociation and photodesorption from water ice range from about 10–3 to 10–1. However, in the case of photodissociation of water in the gas phase, it is close to unity. Exciton dynamics carried out by a quantum mechanical time-dependent propagator shows that in the eight most stable water hexamers, the excitation diffuses away from the initially excited molecule within a few femtoseconds. On the basis of these quantum dynamics simulations, it is hypothesized that the ultrafast exciton energy transfer process, which in general gives rise to a delocalized exciton within these clusters, may contribute to the low efficiency of photolytic processes in water ice. It is proposed that exciton diffusion inherently competes with the nuclear dynamics that drives the photodissociation process in the repulsive S1 state on the sub-10 fs time scale.Journal of Physical Chemistry Letters 11/2012; 3(23):3610–3615. DOI:10.1021/jz301640h · 6.69 Impact Factor