Inelastic intermolecular exchange of vibrational quanta and relaxation of vibrationally excited states in binary solid systems

The processes of molecular relaxation in the binary nitrate–perchlorate solid systems LiNO$_{3}$–LiClO$_{4}$, NaNO$_{3}$–NaClO$_{4}$, and KNO$_{3}$–KClO$_{4}$ have been investigated using Raman spectroscopy. It has been found that the relaxation time of the $\nu_{1}(A)$ vibration of the NO$_{3}^{-}$ anion in the binary solid system is shorter than that in the pure metal nitrates. It has been shown that an increase in the relaxation rate is caused by the existence of an additional mechanism of relaxation of vibrationally excited states of the nitrate ion in the system. This mechanism is associated with the excitation of a vibration of another anion (ClO$_{4}^{-}$), as well as with the “creation” of a lattice phonon. It has been established that the condition for the realization of the relaxation mechanism is that the difference between the frequencies of the aforementioned vibrations should correspond to the range of sufficiently high densities of states of the phonon spectrum.