Dynamics induced by electronic transitions: Finite-temperature relaxation of self-trapped excitons to defects in NaCl

Molecular dynamics simulations of self-trapped exciton relaxation and associated defect formation in 621-atom NaCl clusters provide a useful framework for discussing hypotheses suggested by the recent experimental results of Tanimura and Hess. Their experiments on subpicosecond absorption spectroscopy of NaCl versus temperature showed that a major part of the temperature-dependent F–H defect formation yield in NaCl occurs within a few picoseconds, a time much shorter than the lifetime of equilibrated self-trapped excitons at the corresponding temperatures. This forces a re-examination of mechanisms for temperature-dependent conversion from self-trapped excitons to defect pairs. Molecular dynamics simulations are able to test some suppositions and display dynamic population of characteristic vibrational modes during the relaxation. The results suggest a hypothesis that a particular low-frequency, odd-parity mode of the halogen molecular ion, which is strongly excited and has a long lifetime in the simulation, is important in the dynamics of fast, temperature-dependent defect formation, in the observed slow cooling of STE luminescence in KI, and in anomalous mobility of the “dynamic H-center” observed by Saidoh, Hoshi, and Itoh. A periodic, coherent attempt on the main barrier to defect formation is assisted by thermal noise, and yields a result in phase with the periodic attempt. This concept is describable in the framework of stochastic resonance.