Ultrafast Two-Photon Absorption Approach to Optical Line Shape Measurements

Abstract

An analysis of optical heterodyne detected (OHD) two-photon absorption (TPA) resonant signals in transparent materials is presented. The formal analogies between the vibrationally resonant Raman and electronically resonant TPA P ( 3 ) signals and response functions are given. Fourier transform procedures are shown to allow recovery of the complex TPA response function and eliminate residual Raman contributions from these spatially selected OHD TPA responses. The phase selectivity and wavelength dependence of this technique can be exploited to reveal both the real and imaginary parts of the line shape function describing the solute-solvent response to the two-photon resonant electronic excitation. In-phase and in-quadruture measurements at any resonant wavelength yield the absorption band shape. Intramolecular vibronic structure can be determined for totally diffuse absorption bands. These effects are illustrated by the analysis of two model electronic absorption systems: the MD-simulated methyl iodide B-state origin band in Ar and a multimode Brownian-oscillator-modeled absorption line shape with simple vibronic structure. Electronic excitations less accessible to one-photon resonant ultrafast studies such as UV/VUV transition, electronic excitations in inherently optically dense media, and dipole-forbidden states are well suited for study by this TPA approach.