On neighbouring-level effects, and the validity of the rotating-wave approximation, for a quasi-continuum five-level model doorway system
Sachiko Nakai, A. Brian Yamashita, William J. Meath · Molecular Physics · 1990
A five-level model molecule consisting of a ground and four nearly degenerate excited states, where the excited states arise, via static couplings, from the interaction of a doorway state with three background states, is used to investigate neighbouring-level effects and the validity of the generalized rotating-wave or quasi-resonant approximation for such systems. Calculations are performed to determine the (single-photon) absorption spectra associated with the model, and the underlying temporal behaviour of the molecular states, using both RWA and (exact) Floquet techniques, as functions of the frequency and field strength of the applied sinusoidal electric field. For the field strengths studied, corresponding to ground-doorway-state transition coupling strengths (E d - E g) < 0·04, the RWA results for the overall absorption spectra and for the resonance profiles associated with the individual excited states are generally in excellent agreement with exact calculations, although some interesting subtle differences do occur for the largest considered. On the other hand, the temporal development of the molecular states is well represented by the RWA only for small times. The spectral results, as a function of , are used to discuss some of the interesting features, due to neighbouring-level effects involving the nearly degenerate excited states, that occur during the transition from weak to strong molecule-EMF coupled problems. These include intensity stealing by off-resonance molecular states and inversions of the order of resonance frequencies relative to those corresponding to the weak-field limits.