MOLECULAR MULTIPHOTON TRANSITIONS - COMPUTATIONAL SPECTROSCOPY FOR PERTURBATIVE AND NONPERTURBATIVE REGIMENS

The total Schrodinger equation for an electromagnetic field interactin g with a molecule is shown to lead to time independent or time depende nt coupled differential equations. The time independent equations resu lt from using a quantized representation, i.e., photon number states, of the electromagnetic field. The stationary states of such a quantize d field-molecule system are called dressed states. Appropriate numeric al methods are presented in order to treat radiative and non-radiative interactions simultaneously for any coupling strength, i.e. from the perturbative, Fermi-Golden rule limit, to the non-perturbative regime for both types of interactions. Both bound-bound, bound-continuum and continuum-continuum radiative and non-radiative transitions can be tre ated exactly in the present scheme. The relationship between the quant ized time independent approach and the time dependent semiclassical fi eld method is achieved through consideration of the coherent states of the quantized radiation field. In this limit, multiphoton transitions are more conveniently treated by coupled partial differential equatio ns both in time and space. The time dependent approach is therefore mo re appropriate for very short laser pulses, especially for pulse time durations less than the molecular natural time-scales, in which case s tationary states are ill-defined. Examples of both time-independent an d time dependent calculations are presented. In the first case, cohere nt laser control of multiphoton transitions is illustrated by a time i ndependent, all state, coupled equations method. Finally, high intensi ty direct photodissociation by subpicosecond pulses is presented as an example of laser pulse effects from a time dependent calculation in t he non-perturbative regime, where laser-induced avoided crossings can be created by the pulse itself. The coupled equations methods are in p rinciple exact and can be readily implemented for diatomics and triato mics with current computer technology.