Nonlinear Optical Excitation of the Relaxed Vibronic States of the F Center in KC1

We have conducted the first experiment on two-photon excitation of the F center and obtain new and conclusive information about the structure of the relaxed excited states, the strength of electron-phonon interaction, and the amount of vibronic mixing of $|2s〉$ and $|2p〉$ states, allowing the exact solution of the dynamic problem. A new resonance arising from an additional vibronic mixing (3s-3p) has been found. The values of the 2s-2p splitting and of the fluorescence lifetime from |2p^'〉 have been measured for the first time.