Simulation of two-photon absorption spectra of image by direct calculation

Two-photon absorption (TPA) line strengths for transitions within the image configuration of image have been successfully calculated with several different time-independent perturbation methods. However, attempts to use similar methods to predict TPA line strengths for transitions within the image configuration of image have been much less successful, due to the fact that the low-lying image excited states make straight-forward application of perturbation theory unreliable. In this work we carry out a direct calculation of TPA line strengths that take into account the major interactions for image and image configurations nonpertubatively. We also calculate the linewidths of TPA peaks due to nonradiative relaxation to image energy levels by considering the odd parity vibrations responsible for nonradiative relaxation. Comparisons of calculated TPA line strengths and linewidths with experimental spectra allow improved energy-level assignments. The calculated TPA spectra give a good reproduction of the experimental line strengths, polarization dependences, and linewidths.