Mechanism of vibrational relaxation in molecular solids

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Abstract
The NH and ND(A 3Π) vibrational relaxation rates, and the X 3Σ−?A 3Π spectra, have been investigated in different types of amorphous solid rare gas environments. The vibrational relaxation Hamiltonian is short range, and effectively sees only the nearest neighbors. The relaxation rates are independent of whether the local neighborhood host phonon structure is localized or delocalized. In mixed Ar+Kr hosts, NH* relaxes at the same rate with one or two nearest Kr neighbors as in pure Kr. As the number of Kr nearest neighbors increases, the guest–host interactions, controlling both spectra and relaxation, saturate and are not pairwise additive. This result is qualitatively consistent with gas phase van der Waals complex studies indicating incipient chemical bond formation. The correct local mode theoretical formulation of the solid phase relaxation problem is discussed.