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Notes: The photodissociation of H2S through excitation in the first absorption band (λ≈195 nm) is investigated by means of extensive ab initio calculations. Employing the MRD-CI method we calculate the potential energy surfaces for the lowest two electronic states of 1A‘ symmetry varying both HS bond distances as well as the HSH bending angle. (In the C2v point group these states have electronic symmetry 1B1 and 1A2, respectively.) The lower adiabatic potential energy surface is dissociative when one H atom is pulled away whereas the upper one is binding. For the equilibrium angle of 92° in the electronic ground state they have two conical intersections, one occurring near the Franck–Condon point. Because of the very small energy separation between these two states nonadiabatic coupling induced by the kinetic energy operator in the nuclear degrees of freedom are substantial and must be incorporated in order to describe the absorption and subsequent dissociation process in a realistic way. In the present work we treat the coupling between the two electronic states in a diabatic representation extracting the coordinate-dependent mixing angle from the CI coefficients of the electronic wave functions. The nuclear motion is treated in three dimensions in an exact quantum mechanical approach by propagation of a two-component time-dependent wave packet. The calculated absorption spectra for H2S and D2S satisfactorily agree with the measured spectra. In particular, the calculations reproduce the diffuse structures with energy spacing of about 1200 and 850 cm−1 for H2S and D2S, respectively. Furthermore, the calculated rotational- and vibrational-state distributions of the HS and DS fragments reproduce recent measurements in a convincing way. The photodissociation of H2S is a prototype for very fast electronic predissociation. The photon preferentially excites the binding (diabatic) state. This state, however, is quickly depleted by strong coupling to the dissociative (diabatic) state with the complex finally breaking up into products H and HS. The electronic quenching takes place on the time scale of one internal vibrational period only.Our calculations unambiguously confirm that the diffuse structures superimposed to the broad background are caused by symmetric stretch motion—in the binding state—and not by activity in the bending mode as originally assumed.

Notes: We present the results of a three-dimensional wave packet study on the photodissociation of ClNO through excitation of the first singlet state S1. The calculations employ an ab initio potential energy surface depending on the Cl–N and N–O bond coordinates and the ClNO bending angle. By expanding the wave packet in terms of the eigenfunctions of the NO rotor, the time-dependent Schrödinger equation is transformed into a coupled set of 60 two-dimensional partial differential equations which are solved by discretization on a grid. The wave packet yields the absorption spectrum and all partial dissociation cross sections for producing the NO fragment in a particular vibrational–rotational state (nj). The photodissociation of ClNO via the S1 state is a relatively fast process and the necessary propagation time is on the order of 50 fs. The calculated data agree well with recent experimental results. For the first time, we can directly compare the wavelength dependence of partial photodissociation cross sections for a single vibrational–rotational fragment state state with experiment. Our results suggest that the photodissociation of ClNO(S1) proceeds mainly adiabatically for the vibrational degree of freedom.

Notes: We investigate the photodissociation of highly excited vibrational states of water in the first absorption band. The calculation includes an ab initio potential energy surface for the A˜-state and an ab initio X˜→A˜ transition dipole function. The bending angle is fixed at the equilibrium value within the ground electronic state. Most interesting is the high sensitivity of the final vibrational distribution of OH on the initially prepared vibrational state of H2 O. At wavelengths near the onset of the absorption spectrum the vibrational state distribution can be qualitatively understood as a Franck–Condon mapping of the initial H2 O wave function. At smaller wavelengths final state interaction in the excited state becomes stronger and the distributions become successively broader. Our calculations are in satisfactory accord with recent measurements of Vander Wal and Crim.

Notes: The photodissociation of H2O in the second absorption band (X˜→B˜) is investigated in a completely time-dependent approach. The Schrödinger equation is solved by a time-dependent close-coupling method expanding the two-dimensional wave packet in terms of free rotor states. The vibrational degree of freedom of the OH fragment is fixed and only motion on the B˜-state potential-energy surface is considered. The calculated absorption spectrum exhibits a long progression of diffuse structures, ΔE∼0.1 eV, in very good agreement with the experimental spectrum. The structure is readily explained in terms of a recurrence of the autocorrelation function after about 40 fs. The recurrence, in turn, is attributed to special indirect trajectories which on the average perform one oscillation within the deep potential well before they dissociate into products H+OH. These trajections are "guided'' by so-called unstable periodic orbits which persist to energies high above the H+OH(2 Σ) threshold. The existence of unstable periodic orbits leading to a recurrence of the autocorrelation function gives, for the first time, a consistent explanation of the diffuse structure in the absorption spectrum of H2 O in the second band.

Notes: 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 B˜(1A1) electronic state. Nonadiabatic coupling to the lower states A˜ and X˜ 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 B˜ 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 B˜ 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.