On the importance of permanent moments in multiphoton absorption using perturbation theory

Abstract

Rates for multiphoton absorption by molecules in the regime where perturbation theory is valid are computed. The importance of the contributions to the rates of permanent moments is shown. The results are expressed in terms of generalised Einstein B coefficients which are shown to arise both from quantum electrodynamical and semiclassical arguments; the authors remain within the electric dipole approximation throughout. Explicit and detailed results for two- and three-photon absorption are given for plane-polarised and circularly-polarised light. These are found both for molecules rigidly fixed relative to the light beams and for random orientation of the molecules. In the latter case the rates are presented in terms of the transition and permanent moments in the molecular-body coordinate frame of reference.