Simulation of two-photon absorption spectra of Eu2+:CaF2 by direct calculation

Two-photon absorption (TPA) line strengths for transitions within the 4f7 configuration of Gd3+ 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 4f7 configuration of Eu2+ have been much less successful, due to the fact that the low-lying 4f65d 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 4f7 and 4f65d configurations nonpertubatively. We also calculate the linewidths of TPA peaks due to nonradiative relaxation to 4f65d 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.