Photodissociation dynamics of water in the second absorption band. II. A b i n i t i o calculation of the absorption spectra for H2O and D2O and dynamical interpretation of ‘‘diffuse vibrational’’ structures

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Abstract
We calculated the absorption spectra of H2O and D2O in the second absorption band around 128 nm using a two‐dimensional a b i n i t i opotential energy surface for the B̃(1 A 1) electronic state. Nonadiabatic coupling to the lower states à and X̃ 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 bending‐excitation in the B̃ state with associated bending quantum numbers ν’ 2=1,2,3,... as originally assumed. Because the equilibrium HOH bending angle and the equilibrium H–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 the B̃ state potential energy surface. Within the time‐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 bending a n d stretching 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.