Electronic autoionization and vibrational-state distributions in resonant multiphoton ionization ofH2

We investigate the effects of electronic autoionization on the vibrational branching ratios in resonant multiphoton ionization of ${\mathrm{H}}_2$. Ab initio calculations are performed to obtain the vibrational branching ratios for (3+1) resonant-enhanced multiphoton ionization (REMPI) of ${\mathrm{H}}_2$ via the C ${}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ state. Our calculations include the effects of the dissociative ${}_{}{}^1{}_{}{}^{}\mathrm{\Pi }_{\mathrm{g}}^{}{}_{}{}^{}$(1${\mathrm{\sigma }}_{\mathrm{u}}$1${\mathrm{\pi }}_{\mathrm{u}}$) autoionizing state and properly account for the interference between the direct and the indirect (autoionization) channels. We find that the direct and indirect amplitudes are comparable for excitation via the higher ($v_i$≥2) vibrational levels of the C state. Autoionization greatly enhances the branching ratios for Δv≠0 transitions. These calculations underscore the necessity for a proper treatment of both the direct and indirect contributions in understanding the REMPI of molecules via autoionizing states.