Intermolecular phonon decay mechanism of vibrational relaxation in binary salt systems

Molecular relaxation in the KNO3–Na2SO4, NaNO3–Na2SO4 and NaNO3–NaCH3COO binary salt systems was studied by vibrational spectroscopy methods. The relaxation time of the totally symmetrical ν1(A) vibration of the NO3− anion in binary salt systems was found to be shorter than in pure metal nitrates. This increase in the relaxation rate was explained by the presence of an additional mechanism of relaxation of nitrate ion vibrationally excited states in binary salt systems. The mechanism involves excitation of second anion (SO42− or CH3COO−) vibrations and `creationʹ of a lattice phonon. The condition for the existence of such a relaxation mechanism is the correspondence between the difference of the frequencies of these vibrations and the region of fairly high density of states of the phonon spectrum.