The onset of intramolecular vibrational energy redistribution and its intermediate case: The ν1 and 2ν1 molecular beam, optothermal spectra of trifluoropropyne

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Abstract
Using the optothermal method for molecular beam, infrared spectroscopy, we have measured both the fundamental and first overtone of the acetylenic C–H stretch in CF3CCH. In the fundamental we observe a spectrum which shows only few perturbations. The majority of lines can be successfully fit to a model assuming an anharmonic coupling of the C–H stretch to a single, near‐resonant background state with a coupling matrix element of 0.006 cm−1. We have observed other perturbations in this spectrum, including a state coupled by a weak perpendicular Coriolis interaction. All observed couplings are very weak and local in nature. In the overtone, where the density of background states increases by a factor of 100, we observe a spectrum characteristic of a system in the intermediate case of IVR (intramolecular vibrational energy redistribution). Analysis of the R(0) and P(1) transitions provides a homogeneous IVR lifetime of about 2 ns, which is long compared to lifetimes generally quoted for overtone vibrational relaxation. The root‐mean‐square coupling matrix element in the overtone is about 0.0008 cm−1. The higher J transitions in the overtone suggest that Coriolis interactions are present in the spectrum. The interpretation of these spectroscopic results in the context of IVR is discussed.