The rotating wave approximation, including the incorporation and importance of diagonal dipole moment matrix elements, for infrared multiphoton excitations

Abstract

The incorporation of diagonal dipole moment matrix elements into the rotating wave approximation, for the interaction of many-level molecules with a continuous wave laser, is illustrated with particular emphasis on the multiphoton transitions characteristic of IR molecular excitations. Generalized rotating wave approximation (GRWA) and exact Floquet calculations are carried out for the resonance profiles, and the time evolution of the molecular states, associated with such transitions for the interaction of a laser with two, eight-level anharmonic oscillators modeled on the relatively harmonic, and the very anharmonic HCl and HeH+ molecules, respectively. The calculations are carried out for intensities ranging from 1011 to 1014 W/cm2 and for a wide range of frequencies. These results are used to investigate the validity of the ‘‘new’’ GRWA and the importance of diagonal dipole matrix elements for IR multiphoton excitations as a function of frequency and laser intensity. The discussion also includes examples of the use of the analytic expressions for the molecule-electromagnetic field couplings, in the GRWA, to help interpret/predict the effects of the diagonal dipole moment matrix elements on the resonance profiles, and the dynamics, relative to the results corresponding to these matrix elements being zero.