Band relaxation time of excitons in interaction with acoustic phonons

The energy relaxation time of excitons along the dispersion curve is investigated theoretically under the condition of single-phonon scattering by acoustic lattice oscillations. The relaxation rate and time of the excitation to an arbitrary (lower) value of the wave number k are analyzed for excitons of the lowest bands in Ne, Ar, Kr, and Xe cryocrystals as a function of the exciton-phonon interaction force. The quasiparticle relaxation pattern at T = 0 is investigated with allowance for thermally stimulated scattering by phonons. It is shown that a critical temerpature Tc exists in each crystal, separating the intervals in which the diffusion of free excitons is influenced predominantly by band relaxation and by self-trapping; the value of the critical temperature depends on the height of the exciton self-trapping barrier.