THE HARMONIC 2-COLOR ROTATING WAVE APPROXIMATION - ITS VALIDITY AND AN APPLICATION TO THE EFFECTS OF PERMANENT DIPOLES AND LASER PHASE IN MOLECULAR EXCITATIONS

The harmonic two-colour excitation of a two-level molecule, where the frequencies of the two continuous wave lasers are multiples of a certa in frequency, is studied in the rotating wave approximation (RWA) and by using exact methods. Included are the effects of a non-zero differe nce d between the permanent dipole moments of the two states involved in the transition. For two independent frequencies, the two-colour RWA yields analytical results only when one two-colour resonance dominate s the transition, while for the harmonic analogue, analytic solutions, exhibiting the effects of both laser and molecular parameters, are av ailable for problems involving competing two-colour resonances. Analyt ical solutions for both the time-dependent populations of the excited state, and for the associated resonance profiles, are derived, applied to a model two-level molecule, and tested by comparison with exact re sults obtained using Floquet techniques. The results are used to discu ss the phase control of molecular excitation through the interplay of competing resonances involving the effects of d not equal 0; the compe tition vanishes if d not equal 0. Both fixed molecule-laser configurat ions, and the effects of orientational averaging, are considered.