The dynamics of the formation of self-trapped excitons and Frenkel pairs were studied during band-gap excitation by means of fs laser pulses. The relaxation process of photo-generated electron-hole pairs was found to involve 2 distinct stages. A Frenkel pair which comprised an F center and an H center was formed in the first stage, which terminated within a few ps after excitation. Self-trapped excitons with an off-center configuration were formed only in a second stage, which followed the first stage and continued for over 20ps at low temperatures.

H.Fujiwara, T.Suzuki, K.Tanimura: Journal of Physics - Condensed Matter, 1997, 9[4], 923-36