Nonadiabatic response to short intense laser pulses in dissociation dynamics

Abstract

Molecular nonadiabatic response to the sudden switching on (rise times of few femtoseconds) of intense (tens of TW/cm${}^{2})$ laser pulses may lead to some unexpected enhancements of dissociation rates. These effects are accounted for by the spreading of the molecule-plus-field wave packet over several resonances (instead of a single one predicted when the laser is adiabatically switched on) interfering during the excitation process. Typical time-resolved signatures of such highly nonlinear responses are thoroughly investigated in terms of the evolution of vibrational survival probabilities and fragment kinetic energy spectra of ${\mathrm{H}}_{2}^{+}$ molecular ion taken as an illustrative example. A plausible 532-nm wavelength single-photon dissociation scenario bringing into the system less energy than the minimum required for the fragmentation to occur is examined. The suggested mechanism, which may be termed below-threshold dissociation, as opposed to above-threshold dissociation, refers to a very sharp rise of the laser pulse resulting into temporal excitation of some resonances lying above the single-photon dissociation energy, with efficient decay rates.