ON THE EFFECTS OF ABSOLUTE LASER PHASE ON THE INTERACTION OF A PULSED-LASER WITH POLAR VERSUS NONPOLAR MOLECULES

The absolute laser phase dependence of the time-dependent populations of the molecular states, including the steady-state (long time) popula tions of the states, associated with the interaction of a molecule wit h a pulsed laser is investigated using illustrative two-level examples . One-photon transitions, including the effects of permanent dipoles, are discussed as a function of the pulse duration, intensity, and (abs olute) laser phase, for selected laser frequencies. The effects of las er phase can be large, depending on the values of the pulse duration f or a given frequency and intensity. The effects of permanent dipoles, relative to no permanent dipoles, are significant for large laser fiel d strengths epsilon(0). When the laser-molecule coupling parameter b = mu(12)epsilon(0)/E-21 greater than or equal to 0.2, where mu(12) and E-21 are the transition dipole and energy difference between the groun d and excited states, respectively, the dynamics of the pulse-molecule interaction are (strongly) phase dependent, independent of pulse dura tion, whereas the corresponding steady-state populations of the molecu lar states may or may not be phase-dependent depending on the pulse du ration. Analytical rotating wave approximations for pulsed laser-molec ule interactions are useful for interpreting the dynamics and the stea dy-state results as a function of field strength and pulse duration, i ncluding the effects of permanent dipole moments. The results reported in this paper are based on molecular parameters associated with an S- 0 --> S-1 electronic transition in-a dipolar molecule. However, they a re presented in reduced form and therefore can be scaled to other regi ons of the electromagnetic spectrum. Short, intense pulses at or beyon d the limits of current laser technology will often be required for th e types of absolute laser phase effects of this paper to be appreciabl e for electronic excitations. The discussion, in the UV-VIS, also suff ers from the use of a two-level model and from the requirement of fiel d intensities that can be beyond the Keldysh limit. For other spectral regions, these absolute laser phase effects will be much more readily applicable. (C) 1998 American Institute of Physics. [S0021-9606(98)00 945-3].