Two-color multiphoton transitions in molecular beam electric resonance studies: Rotating wave versus Floquet, and on- versus off-resonance, calculations

Abstract

A nonzero difference, d, between the diagonal dipole moment matrix elements, μjj, of two molecular states involved in either one- or two-color multiphoton transitions, can have substantial impact on the temporal evolution and spectral behavior of the states. The effects of d≠0 are investigated in this paper for two-color transitions in a two-level system previously studied in one-color molecular beam electric resonance (MBER) experiments on symmetric top molecules. The calculations suggest a two-color analog to the one-field experiments, where the flexibility furnished by the field parameters of the two continuous wave electric fields, including relative phase, can be used to advantage. Both exact Floquet calculations and the rotating wave approximation (RWA) are used in this study. Analytic RWA expressions for the one- and two-color molecule-laser(s) couplings are particularly useful in helping to interpret and/or predict the effects of d≠0. The novel aspects of two-color laser-molecule interactions, relative to the one-field case, are emphasized. In addition to investigations related to MBER studies, this work contributes to the more formal aspects of two-color laser-molecule interactions. It is shown that very useful analytical two-level RWA solutions for the on-resonance temporal behavior of the molecular states are available, even in the presence of competing resonances, whereas off-resonance numerically useful analytical results are available only when one multiphoton resonance dominates a transition.