Isotope effects in laser-induced multiphoton molecular dynamics

Optical nonlinearities affecting absorption spectra, above threshold dissociation, bond softening, and vibrational trapping are among the most important processes that have so far been examined when describing the dynamics of multiphoton excitation and fragmentation of molecular systems subject to intense laser fields. The relative merits of time-dependent and independent approaches are overviewed and discussed within the framework of H2+ photodissociation for which recent experimental data are available. Isotopes H2+ – D2+ behave in a markedly different manner with respect to the radiative coupling. Some of these differences are exploited for the detailed understanding of the underlying mechanism of the bond softening process. It is shown that this is not merely a single photon potential barrier lowering mechanism as has previously been invoked, but a competition between this mechanism and another five photon exchange mechanism, namely the simultaneous absorption of three photons followed by the subsequent emission of two photons. As for the vibrational trapping process, the relative sensitivity of the isotopes to the laser characteristics is such that, by adequately adjusting the wavelength and the intensity, one can suppress the fragmentation of one of the isotopes by locally confining it, whereas the other dissociates very fast. This is used as a possible scheme for isotope separation using intense lasers.