MULTIPHONON DECAY OF STRONG MODE IN QUANTUM-LATTICE

A nonperturbative theory of multiphonon anharmonic decay of strongly e xcited local mode is developed whereby the mode is considered classica lly and phonons, quantum mechanically. The decay rate of the mode is e xpressed via the negative frequency parts of the phonon pair correlati on functions. In the case of two-phonon decay the later satisfy the li near integral equations while in the case of two-and more-phonon decay they satisfy the nonlinear integral equations. As a result, the proce sses mentioned dif ferently depend on the mode amplitude ii: two-phono n processes smoothly deminish if A --> infinity while three- and more- phonon processes are fully switched-off at large amplitudes and they a bruptly switch-on if the amplitude approaches the critical value. At t hat the decay rate gets rather high value (of the order of the mode qu antum per period). The final stage of the relaxation is well described by the perturbation theory.