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Chemical Physics Letters

On the competition between permanent dipole and virtual state two-photon excitation mechanisms, and two-photon optical excitation pathways, in molecular excitation

B. N. Jagatap, William J. Meath

Published Aug 1, 1996• 32 citations
Publisher
DOI
10.1016/0009-2614(96)00620-3

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Role of permanent dipoles and orientational averaging in the phase control of two-color, simultaneous one- and three-photon molecular excitations

Alex Brown, William J. Meath

The effects of permanent dipole moments and those due to the randomness of molecular orientation in the phase control of molecular excitation are discussed for the simultaneous one- and three-photon excitation of a two-level model molecule. In this transition scheme both transitions can occur with or without the presence of permanent dipoles and the results are contrasted to those corresponding to the one- and two-photon excitation of a two-level molecule, which requires the presence of permanent dipoles. The dependence of the temporal evolution of the excited state and the associated resonance profiles on the relative phase of the lasers is used to monitor the control of the excitation process. Analytical perturbation theory, the rotating-wave approximation, and exact Floquet results for those observables are used for this purpose. Both fixed molecule-laser configurations and situations where the absorbing molecules assume random orientations with respect to the laser beams are considered. \textcopyright{} 1996 The American Physical Society.

Physical Review A•Apr 1, 1996•18 citations
One of the purposes of this paper is to develop an analytical many-level generalized rotating wave approximation (GRWA), including the effects of permanent dipoles, for the excitation of many-level molecules through the simultaneous absorption of two photons. Included are expressions for the two-photon laser–molecule coupling C, and its two components Cd and Cv corresponding to the direct permanent dipole and the virtual state excitation mechanisms, respectively, and related observables such as the time-dependent populations of the initial and final states of the excitation process and resonance profiles. This GRWA treatment also includes an energy shift parameter e, which causes shifts in the position of the resonance energy as the laser intensity increases. The effects of permanent dipoles are very different in Cv and e versus Cd. These effects have been discussed previously for Cd using analytic two-level RWA approaches. The analytical results for Cv and e obtained here are new as is their use in discussing the influence of permanent dipoles in the parts of the two-photon excitation process involving virtual states. In the absence of permanent dipoles Cd is zero whereas Cv and e are not; they equal the corresponding perturbation theory results. The GRWA and perturbative results are related by Bessel function damping functions which, for dipolar molecules, damp out the divergence of the perturbative results as the laser intensity increases. Illustrative examples are given for a two-photon excitation involving a model chromophore which has a significant virtual state contribution through both the laser–molecule coupling and the energy shift parameter.
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.
The effects of permanent dipole moments and those due to the randomness of molecular orientation in the phase control of molecular excitation are discussed for the simultaneous one- and three-photon excitation of a two-level model molecule. In this transition scheme both transitions can occur with or without the presence of permanent dipoles and the results are contrasted to those corresponding to the one- and two-photon excitation of a two-level molecule, which requires the presence of permanent dipoles. The dependence of the temporal evolution of the excited state and the associated resonance profiles on the relative phase of the lasers is used to monitor the control of the excitation process. Analytical perturbation theory, the rotating-wave approximation, and exact Floquet results for those observables are used for this purpose. Both fixed molecule-laser configurations and situations where the absorbing molecules assume random orientations with respect to the laser beams are considered. \textcopyright{} 1996 The American Physical Society.