Resonance-enhanced two-photon dissociation ofH2through nonadiabatically coupled intermediate and final states

Abstract

Nonperturbative time-dependent calculations of the resonance-enhanced two-photon dissociation probability of ${\mathrm{H}}_{2}$ in two frequency laser fields from the ground $X{}^{1}{\ensuremath{\Sigma}}_{g}^{+}(v=0,j=0)$ level to the final continua of $\mathrm{GK}{}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ and $I{}^{1}{\ensuremath{\Pi}}_{g}$ states have been made as functions of the laser frequencies. The two fields are taken to have linear parallel polarizations with identical sine-squared time dependences of the amplitudes. The first field of frequency ${\ensuremath{\omega}}_{1}$ is near resonant with the two closely spaced excited intermediate levels $B{}^{1}{\ensuremath{\Sigma}}_{u}^{+}(v=14,j=1)$ and $C{}^{1}{\ensuremath{\Pi}}_{u}^{+}(v=3,j=1),$ which are strongly coupled to each other through nonadiabatic interaction as well as by radiative Raman coupling. Thus two intermediate levels with mixed ${\ensuremath{\Sigma}}^{+}\ensuremath{-}{\ensuremath{\Pi}}^{+}$ character are created. The molecule finally dissociates through coherent excitation to a number of near-resonant discrete rovibrational bound levels of $H\overline{H}{}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ and $J{}^{1}{\ensuremath{\Delta}}_{g}^{+}$ embedded into the continua of GK and I states as well as by direct transition to these continua, by absorption of a second photon of frequency ${\ensuremath{\omega}}_{2}.$ The nonadiabatic interactions of the bound levels of $H\overline{H}$ and J states, with the respective continuum of GK and I states, give these levels a predissociating character. The interference of the direct transition amplitudes to the continua, and those through the various overlapping predissociating resonances, gives rise to a resultant structure in the dissociation probability with the variation of ${\ensuremath{\omega}}_{2}.$ The bound levels used are either (a) a group of three closely spaced vibrational-rotational levels, $H\overline{H}{}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ $(v=4,$ $j=0$ and 2), $J{}^{1}{\ensuremath{\Delta}}_{g}(v=4,j=2);$ and (b) the next group of three closely spaced levels, $H\overline{H}{}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ $(v=5,$ $j=0$ and 2), $J{}^{1}{\ensuremath{\Delta}}_{g}(v=5,j=2).$ Far from resonance with any predissociating level, the dissociation probability becomes equal to the value obtained by considering only the direct transitions to the continua. For different fixed values of ${\ensuremath{\omega}}_{1},$ the variation of the dissociation probability against ${\ensuremath{\omega}}_{2}$ reflects the characteristics of the excitation of the intermediate resonances with mixed B and C character. On or near resonance, the excitation of the predissociating levels of the J electronic state plays a crucial role in determining the dissociation line shape, whereas the excitation of the predissociating levels of the $H\overline{H}$ electronic state do not affect the dissociation line shape significantly.