Specific features of optical and nonradiative transitions in molecular systems in the field of an intense electromagnetic wave are studied with account of squeezed vibrational states. It is shown that a field of laser radiation considerably enhances the tunneling processes in electronic vibrational systems and makes it possible to obtain the generation of higher harmonics. Conditions are found under which squeezed vibrational states can lead to some stabilization of excited electronic states with respect to the processes of luminescence.
A mechanism is proposed for a nonradiative transition between excited energy levels of the NV– center in diamond. The nonradiative transition between the levels of the spin triplet and the singlet is possible due to the spin–orbit interaction with simultaneous excitation of vibrational (vibronic) states of the cluster and the excitation of lattice phonons and, hence, can be obtained in the second order of perturbation theory. For describing this process, the spin–vibronic interaction operator and nonzero matrix elements ensuring nonradiative transitions are obtained. It is found that the matrix elements are proportional to the overlap integrals for one-particle electron wavefunctions. It is shown that the rate of transitions 3E → 1A1 and
1E~ → 3A2 are described by identical expressions. Therefore, the nonradiative transitions considered here are explained not by a multiphonon process, but by the excitation of vibronic states of atoms of the NV– center. Numerical estimates are in good agreement with experimental data.