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

Abstract

We calculated the absorption spectra of H2O and D2O in the second absorption band around 128 nm using a two-dimensional ab initio potential energy surface for the B̃(1A1) 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 and 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.

Communication: State-to-state differential cross sections for H2O($\tilde B$B̃) photodissociation

“…[21][22][23][24][25] Due to the large torque along the dissociation path, the OH(X) fragment formed via non-adiabatic transitions was found to be rotationally excited, in agreement with experimental observations. These conclusions were confirmed by a later wavepacket study on three-dimensional PESs by van Harrevelt and van Hemert, 27, 31 who reported not only the absorption spectrum, but also ro-vibrational distributions of the OH fragments as well as the electronic branching ratio.…”

“…As discussed before, these undulating peaks in the absorption spectrum stem from wavepacket recurrence in the Franck-Condon region. 21,22,31 In Fig. 2, we compare our calculated DCSs with those measured in a recent experiment.…”

“…For this reason, it has attracted intense experimental [9][10][11][12][13][14][15][16][17][18][19] and theoretical attention. 14,[20][21][22][23][24][25][26][27][28] The photodissociation of H 2 O in its B band is initiated by photoexcitation from its ground (X 1 A ) state to its second excited state (B 1 A ). The earlier studies on the potential energy surfaces (PESs) established the existence of two conical intersections (CIs) between the two electronic states, which facilitate dissociation into both the H + OH(X) and H + OH(A) channels.…”

Photodissociation of water. II. Wave packet calculations for the photofragmentation of H2O and D2O in the B̃ band

“…Several approximate twoand three-dimensional dynamical calculations, using either the adiabatic or the diabatic potential energy surfaces, have been reported. 8,9,19,[21][22][23][24][25] Whereas these studies have successfully explained the rotational distributions of OH(X 2 ⌸) and OH(A 2 ⌺ ϩ ), the calculated absorption spectra are not in satisfactory agreement with experiment, 12 and the relative fractions for different products OH(X), OH(A), H 2 ϩO( 1 D), and O( 3 P)ϩHϩH, have not yet been studied. A nonadiabatic treatment including all three internal coordinates is crucial to obtain even only qualitative agreement with experiments.…”

Vibrational Revivals and the Control of Photochemical Reactions

“…54,55 Note, however, that the anisotropy of the electronic B 1 A 1 state of water is very large around the angle γ ) 104°so that for the B r X transition the rotational degree of freedom cannot be neglected. 56,57 Let us assume that a wave packet in the state |g〉 is prepared. This can be achieved, for example, by strong-pulse infrared excitation or multiphoton transitions via excited electronic states, see section 3.…”