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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.

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.