NONEQUILIBRIUM PHONON DYNAMICS IN LAF3-PR3+

The presence of low-lying optical modes in LaF3 can introduce addition al decay processes which cause the frequency dependence of the phonon relaxation rate to deviate from the omega(5) dependence predicted by t he isotropic dispersionless model. In this paper we investigate the fr equency dependence of the phonon relaxation rate in LaF3:Pr3+ over a w ide range of frequencies using a combination of defect-induced. one-ph onon absorption and absorption vibronic sideband spectroscopy. For com parison with our experimental results, we calculate the phonon relaxat ion rates for a system with low-lying optical modes. In our model, we consider that both the dispersion curves and the three-phonon coupling constants are isotropic. (C) 1998 Elsevier Science B.V. All rights re served.