The mechanism of vibrational relaxation in solids: Multiphonon relaxation of O2(c 1Σ−u) in Ar, Kr, and mixed Ar–Kr matrices

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Abstract
The vibrational relaxation of 16–18O2(c 1Σ−u) has been directly time resolved in Ar, Kr, and mixed Ar–Kr matrices. A vibrational cascade (v=2→1→0) is produced following near resonant intersystem crossing from v=0 C 3Δu. The vibrational relaxation rates slow in Kr host despite the fact that O2 (c 1Σ−u) is more strongly solvated in Kr. In solid Kr, relaxation is slower than host induced fluorescence. The C 3Δu→c 1Σ−u intersystem crossing rate, the spectral shift, and the vibrational relaxation rates are monitored in mixed matrices as the environment changes from pure Ar to pure Kr. The spectral shift shows partial saturation, or weak complex formation, behavior; the vibrational relaxation behavior is consistent with pairwise additive forces. The relaxation rates are strongly temperature dependent. It is suggested that O2(c 1Σ−u), as well as other first row diatomics such as NO(a 4Π) and C−2(a 4Σ), relax via a direct multiphonon mechanism. An analogy with vibrational predissociation in gas phase van der Waals clusters is considered. Symmetric and antisymmetric multiphonon processes are considered in terms of an earlier pseudotriatomic complex model. An isotopic test for symmetric vs antisymmetric multiphonon relaxation is proposed and applied to C−2.