The effect of electronic autoionization on vibrational branching ratio in (1 + 1')-photon resonance-enhanced multiphoton ionization of molecules: dependence on laser intensity

Abstract

We have studied the laser intensity effect on the vibrational branching ratio (VBR) in -photon resonance-enhanced two-photon ionization of molecules, considering the electronic autoionization through the lowest autoionizing (AI) state of symmetry. In this ab initio calculation we have studied resonant two-photon transitions from the ground state to the doubly excited autoionizing state of symmetry as well as to the electronic continuum (:) of molecules via the resonant intermediate levels. Transitions to two different continuum energies (-0.4500 au and -0.5853 au) have been considered; the first one is 1.36 eV above the dissociation threshold (-0.50 au) of the ion and the second one is 0.048 eV above the vibrational level of the ion in the ground state . Parallel transitions through other intermediate near-resonant rovibrational levels v = 0-3 and 5-20 with the allowed values of total angular momentum j of the state have been included in the calculation. Besides the effect of interference of direct ionization with autoionization channels, we have considered here (i) the effects of mutual interference among parallel autoionization channels and that among parallel ionization channels via the near-resonant and resonant rovibrational levels of the state and (ii) the effect of the two-photon coupling between intermediate rovibrational levels via the autoionizing state and the continuum in order to study the laser intensity dependence of vibrational distribution of ions in the ground state. It has been shown that these two effects (channel interference effect and two-photon coupling effect) lead to non-Franck-Condon type VBR and the values differ from those obtained by considering the direct ionization channels alone. The laser intensity dependence of VBR is also different from that obtained by neglecting the autoionization channels. For transitions to two different continuum energies these effects become prominent at two different laser intensities. Furthermore, it has been found that the transitions through different intermediate resonant rotational levels of the state give rise to different vibrational branching ratios.