Infrared fluorescence study on the threshold of intramolecular vibrational state mixing

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Abstract
We have performed an infrared fluorescence study on the intramolecular vibrational relaxation (IVR) from C–H stretch fundamentals in isolated ground state polyatomic molecules. It was found that a density of about 70 combined rotational–vibrational states of a common J and symmetry species per cm−1 is needed to ensure that complex state mixing and IVR occur, although we observed a substantial range of couplings for initial states of the same molecule at higher state densities above the threshold. Propynal was studied as an example of a molecule with a low state density. Fluorescence from propynal shows that although there is substantial state mixing, it is best described as a complicated case of Fermi resonance and Coriolis coupling, as in smaller molecules rather than the multistate mixing seen in larger ones. Infrared fluorescence from terminal C–H stretches of some acetylene derivatives was also measured; these show a slight difference in the extent of IVR from other C–H stretches. Molecules were cooled and isolated in supersonic molecular beams and excited in the C–H stretch region with an optical parametric oscillator. Fluorescence spectra were recorded with either a circular variable filter machine or a cryogenic Michelson interferometer.