Abstract

The change in electronic wavefunction brought about by optical excitation of a color center causes the surrounding ions to seek a new configuration of lower energy. Thus, optical excitation creates a momentary high level of localized vibrational excitation which the center must lose before it can emit in a Stokes shifted band. There are two schools of thought about how the vibrational relaxation takes place. One holds that the "configuration coordinate" (the most significant normal mode of ionic motion) must undergo a number of cycles of damped motion, shedding the excess energy in a sequence of optical phonons. The other [1] holds that the relaxation can take place in a fraction of the period of one optical phonon, and that the excess energy is released all at once in a shower of coherent phonons. Previous studies of impurity centers with only moderate coupling between electronic and ionic parts [2], or of a color center with a very large Stokes shift [3] seemed to indicate the former model, as the measured low temperature relaxation times were always many times the phonon period. However, we present here the first evidence that a color center (in this case the ${\text{F}}_2^{+}$, a single electron trapped by a pair of adjacent anion vacancies in an alkali halide crystal) relaxes according to the latter model. That is, we have made measurements indicating a relaxation time of less than one phonon period for that center.

© 1986 Optical Society of America