The dependence of the population of the target state, and the excitation cross section and rate, on pulse duration Q, laser intensity I, and a molecular matrix element A is discussed for two-photon molecular excitation. Perturbative and rotating wave approximation (RWA) expressions for the observables are obtained; the latter are used to discuss the validity of the former. For example, the perturbative cross section and rate increasingly underestimate the RWA results as I increases for given A and Q. Two- and ten-level model dipolar molecules are employed for illustrative purposes. The results are relevant for understanding two-photon excitation processes and their enhancement and include discussions of the roles the permanent dipole- and virtual-state mechanisms in such processes and of the validity of using intensity-independent cross sections to gauge the strength of such excitations.
© 2008 Optical Society of America
Original Manuscript: December 11, 2007
Revised Manuscript: February 25, 2008
Manuscript Accepted: February 25, 2008
Published: April 30, 2008
William J. Meath, "Enhancement of the simultaneous absorption of two photons for pulsed laser-molecule interactions," J. Opt. Soc. Am. B 25, 865-876 (2008)