Photodissociation dynamics of water in the second absorption band. II.Abinitiocalculation of the absorption spectra for H2O and D2O and dynamical interpretation of lsquo;lsquo;diffuse vibrationalrsquo;rsquo; structures

摘要

We calculated the absorption spectra of H2O and D2O in the second absorption band around 128 nm using a twohyphen;dimensionalabinitiopotential energy surface for theBtilde;(1A1) electronic state. Nonadiabatic coupling to the lower statesAtilde; andXtilde; and the vibrational degree of freedom of the OH fragment are completely neglected. Despite these limitations the agreement with the measured spectra is very satisfactory. The overall shape, the width, and the energetical position of the maximum are well described. Most important, however, is the reproduction of the diffuse vibrational structures superimposed on the broad background. It is demonstrated that this structure is not caused by pure bendinghyphen;excitation in theBtilde; state with associated bending quantum numbers ngr;rsquo;2=1,2,3,... as originally assumed. Because the equilibrium HOH bending angle and the equilibrium Hndash;OH distance are very different in the ground and in the excited state, the main part of the spectrum and especially the diffuse structures occur at high energies within the continuum of theBtilde; state potential energy surface. Within the timehyphen;dependent approach, based on the autocorrelation function and simple classical trajectories, it is shown that the diffuse structures originate from the temporary excitation of a large amplitude bendingandstretching oscillation embedded in the continuum (short lived quasiperiodic orbits). The vibrational period of this mode is approximately 40 fs and the lifetime of the trapped trajectories is on the average one vibrational period.