Nonadiabatic effects on resonance-enhanced two-photon dissociation ofH2

Resonance-enhanced two-photon dissociation from the ground ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{+}{}_{}{}^{}$ (${\mathit{v}}_{\mathit{g}}$=0,${\mathit{J}}_{\mathit{g}}$) state of ${\mathrm{H}}_2$ to the final continua of EF, GK, and I states has been studied using both linear parallel and crossed polarizations of the two-photon fields. The first field creates an aligned population in the intermediate resonant state B ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{u}}^{+}{}_{}{}^{}$ (${\mathrm{v}}_{\mathrm{i}}$=0,3,${\mathit{J}}_{\mathit{i}}$) for which the reduced density-matrix description has been adopted. This intermediate state is dissociated through coherent excitation of a number of near-resonant discrete vibrational levels, bound and quasibound, of HH¯ and I, respectively, as well as by direct transition to the continua of EF, GK, and I by absorption from the second field. The nonadiabatic interaction of the HH¯ and I levels with the continuum of GK gives these states predissociating characteristics and the whole process can be formulated in terms of overlapping Fano resonances. The nonadiabatic couplings of the continua of I and EF with the HH¯ bound levels have been neglected because of their smallness. The interference of the transition amplitudes to different overlapping predissociating resonances gives the resultant structure in the dissociation probability which is studied for the second photon wavelength covering a range between the dissociation threshold of the final states and the ionization threshold of ${\mathrm{H}}_2$. The dissociation cross section obtained from these nonadiabatic calculations are found to be drastically altered from the adiabatic and Born-Oppenheimer results.