Intramolecular vibrational relaxation from C–H stretch fundamentals

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Abstract
Infrared fluorescence measurements have been used to determine the presence or absence of intramolecular vibrational relaxation (or more precisely state mixing) from an excited C–H stretch in 23 representative molecules varying in size from methane to nonbornene. Included are aliphatic hydrocarbons, aromatic molecules, ethers, and ketones, including cyclic molecules. The rate of resonance fluorescence, calculated relative to a nonrelaxing molecule, was used as the criterion of relaxation. Small rates imply state mixing and hence in the large molecule limit, relaxation. The primary correlation with the presence of mixing is with state density; about ten states per wave number (cm−1) are needed to insure mixing. The spread in threshold densities reflects variations in anharmonicity. For some molecules true relaxed (non C–H stretch) fluorescence is observed. These experiments were done in a molecular beam, using a pulsed optical parametric oscillator for excitation and a circular variable filter for spectral resolution.