Ultrafast electron-lattice relaxation of optically excited centers in crystals

The excited-state dynamics of a localized center (defect, molecule, cluster) in a nonconducting crystal is discussed, and the Pulse Model is introduced to study its temporary behavior on a scale from few femtoseconds to tens of picoseconds after ultrafast electronic excitation. The simple Pulse Model and a modified version, both elaborated few years ago by the authors, allow a numerical selfconsistent evaluation of the electron-phonon coupled dynamics during the relaxation, i.e. the values of position and momentum of the ions as well as of the wavefunction and energy of the electron as function of time. In particular the process of relaxation of the ions surrounding the center is related to the motion of the multimode wavepacket of lattice phonons generated by the ultrafast optical excitation of the center Numerical data for defects in alkali halides in the limit of strong electron-phonon coupling are presented in two cases: that of a non degenerate electronic excited state and that of a double degenerate excited state (Exe Jahn-Teller case). In the former case, we show that the decay time to the relaxed configuration is related to the autocorrelation time of the multimode phonon wavepacket coupled to the excited electron. In the second case, the Jahn-Teller case, we observe a process of dynamical trapping and hopping between distorted lattice configurations with features depending on the symmetry, electron-phonon coupling strength and temperature.