The results of an experimental and theoretical study of the resonant secondary emission spectra at 4.2 K of wide gap ionic insulators (lattice of the NaCl type) are presented. The well developed character of the free exciton luminescence and the multiphonon resonant Raman scattering spectra (which contain mainly L O lines) point to Frolich intraband scattering of free excitons (polaritons) on LO phonons. It is established that the ratio of the scattered line intensities of the first and second order has a square dependence on the scattered polariton wave vector. For the spectral range 4.97-3.30 eV the dispersion of the scattering cross-section with participation of LO(r) phonons has been measured for the first time. A resonant enhancement of the multiphonon scattering line intensities on the LO(r), LO(X) and TO(r) phonons has been observed. Calculation of the dispersion of the scattering cross-section has been performed using a theoretical model taking into account the scattering of hydrogen-like excitons on LO phonons. Analysis of the cross-section leads to the conclusion that in the excitation frequency range w > Ef: -4R the main contribution to u(w) comes from the discrete excitonic states, but for w < E, -4R the states of the electron-hole continuum play the main role. Agreement between theory and experiment has been achieved by taking into account the electronic damping. The influence of the excitation intensity on the structure of resonant Raman scattering spectra in LiH crystals is also discussed.

No abstract

Starting from the Kubo formula the thermal conductivity is expressed by the two‐phonon Green's function. The latter is calculated for a cubic lattice containing a small fraction of impurities with taking into account the change of force constants. It is shown that the procedure of the configurational average introduces a correlated motion of two phonons which is described by the vertex correction. This correction appears to be essential if the impurity generates at least two resonant modes with close frequencies but with opposite parity. In this case the scattering cross‐section is decreased by the vertex correction. The correlation in the phonon motion is expected to be essential in KI: Ag.

The interference of the anharmonic and impurity scattering of phonons is investigated. In particular, a general formula is obtained for the resonant scattering of phonons by impurities taking into account the inelastic channel which is responsible for the temperature dependence of the total cross-section of impurity scattering. It is found that the anharmonic scattering is decreased by interference effects. The latter is connected with the finite lifetime of the phonons scattered b y impurities. The case of resonant scattering of phonons by isotopic defects is considered in detail. PaccMoTpeHa mwep(PepeHqim aHrapMoHwrecKoro