Nichtadiabatische Effekte bei der Photodissoziation von H_2S

The photodissociation of H_2S through exitation in the first absorption band is investigated by means of extensive ab initio calculations. Employing the MRD-CI Method the potential energy surfaces of the first two excited states of A''-symmetry is calculated. The lower adiabatic potential 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. Because of the very small energy separation between these two states nonadiabatic coupling must be incorporated in order to describe the photodissoziation process in a realistic way. In the present work the coupling is treated in a diabatic representation extacting the mixing angle from the CI coefficients of the electronic wavefunction. The nuclear motion is treated by propagation of a two-component time dependent wave packet. The calculated spectra for H_2S and D_2S satisfactorily agree with the measured spectra. Furthermore the calculated rotational- and vibrational-state distributions of the HS and DS fragments reproduce recent measurements in a convincing way. Looking at the time evolution of the wave packet on the two potential surfaces, the dissociation process of H_2S runs as follow: The photon preferentially excites the binding state. This state is quickly depleted by strong coupling to the dissociative state with the complex finaly breaking up into products HS and H. (orig.)Available from TIB Hannover: RA 1396(1993,7) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman