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It is shown that the neutral deep level state of transition metal impurity in A 2 B 6 semiconductors plays a role of amphoteric localization centre for the excitons: it can capture either electron or hole to the d-shell leaving either hole or electron in a loosely bound hydrogenlike orbit respectively. The physical properties of both excitonic states are compared and the possibility to interpret in these terms the optical spectra of ZnS(Se) doped by Ni and Co is discussed.

The calculation of the radical formation rate constant in solid organic compounds is carried out. This formation is a result of the cleavage of the hydrogen atom from the carbon atom via a radiationless electronic transition in an electronically excited molecule. The electronic transition is supposed to be stipulated by the nonadiabaticity operator and the perturbation theory with respect to this operator is supposed to be applicable. The vibrational interaction of an electronically excited molecule with the host environment is assumed not to be small. The calculation is carried out within the framework of a model in which the whole molecule is represented by a CH group this being treated as a Morse oscillator. The specific features of the different substances in such a model are simulated by different values of the CH bond parameters.

Consistent calculations of the rate constant of radiationless electron transition are reported for a diatomic molecule in a crystal at zero temperature. The transition between electronic states occurs due to nonadiabaticity. Relaxation of the vibrational energy is stimulated by the interaction of the molecule with the crystal. The calculations have been carried out under the assumption that perturbation theory with respect to the nonadiabaticity operator is applicable. The vibrational interaction of the molecule with the environment is assumed not to be small. The electronic terms of the molecule are approximated by Morse potentials.

Radiationless electronic transitions in polyatomic molecules are studied assuming quasidiscrete vibrational spectra. As is shown, the existence of Fermi resonances in such systems allows one to understand irreversible radiationless transitions in isolated molecules. Applying this approach to a molecule in a condensed host material one manages to understand the causes of the early onset of the temperature dependence in the rate constant for radiationless transitions in a number of aromatic compounds.

Double-well potential (DWP) is simulated in a cluster of SiO 2 . The Si-O-Si chain is stretched until a DWP arises. Its parameters vs stretching are investigated. Some considerations in favour of two types of DWP in glasses are put forward.

The diagonal density matrix elements are found for a dynamical subsystem which is either anharmonic or harmonic highly excited oscillator interacting with the crystal vibrations. For the extreme case of the infinitely highly excited oscillator the result is exact. Different limiting cases are considered. In particular for the harmonic oscillator the validity criterion for the balance equation is found.

The cross section for inelastic neutron scattering by a diatomic molecule is calculated. Scattering accompanied by electronic excitation of the molecule is shown to be more probable, under certain conditions, than that accompanied by purely vibrational excitation. The possibility of a neutron-stimulated tunneling transition of the molecule is considered.

The influence of the phonon fluctuations on the equilibrium parameters of the TLS potential curve is investigated. The distribution range of the transparency parameter of the TLS potential barriers is restricted. The effect of the phonon fluctuations results in a strong reduction of the second maximum temperature of the ultrasonic attenuation, which markedly improves the agreement between the theory and experiment.

A theoretical approach is presented intending to unify tunnel model and structural defect concepts existing in the theory of amorphous solids. Origins of double-well potentials (DWP) are discussed and two types of DWPs-soft and rigid-are emphasised. Soft DWPS are mainly responsible for low-temperature properties whereas rigid DWPs determine the properties of glasses at higher temperatures. Tunnelling in rigid DWPs is largely influenced by the barrier fluctuative preparation, the multi-phonon processes are of importance at higher temperatures. The DWP relaxation time up to room temperature is analysed. The sound attenuation due to the relaxation and multi-phonon mechanisms is calculated and their relative contributions are compared. The role of these processes in the thermal conductivity and temperature plateau formation is discussed.

An explanation is proposed for the unusual behaviour of the pressure dependent deep level position in Cr-doped InP. It is shown that this level is attached to the subsidiary L-minimum of the conduction band due to the large contribution of the L 1c -point vicinity in the electron density of states, and the symmetry properties of d-impurity states suppressing the hybridization with the band states in the central ⌝ 1c valley of the conduction band.

The effect of the environmental vibrations on the tunneling in the two- level systems in glasses is discussed. It is shown that the temperature, frequency and other dependences of the tunneling constant are charged drastically due to this mechanism. This allows one to explain the peculiar temperature dependence of the ultrasonic attenuation in glasses in a wide temperature range.