ROTATIONALLY-RESOLVED OVERTONE SPECTROSCOPY OF THE ACETYLENIC C-H STRETCH IN HCN AND $CH_{3}CCH$ IN He CLUSTERS, WHERE DOES THE RELAXATION COME FROM?
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Date
1998
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Ohio State University
Abstract
We have built a new helium cluster isolation spectrometer for IR overtone spectroscopy and here report the first results obtained with it. Rotationally-resolved infrared spectra of the acetylenic C-H stretch first overtones of HCN and $CH_{3}CCH$ embedded in He clusters $(T=0.4 K)$ have been obtained. The use of a resonant power build-up cavity enhances the laser intensity effectively seen by the molecules by a factor of $10^{3}$. This has allowed us to obtain S/N ratios in excess of $10^{2}$, even in the presence of considerable broadening. Because of the low temperature, only one line could be seen for HCN, while for $CH_{3}CCH$ at least three lines in both the P and R branches have been resolved. Although Intramolecular Vibrational Relaxation (IVR) is not expected for HCN, and is probably not contributing in the case of $CH_{3}CCH$ either, measured linewidths range from 1 to 2 GHz. Vibrational, and to a minor extent rotational, relaxation into the superfluid medium is believed to be the cause of the broadening.
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Author Institution: Department of Chemisty, Princeton University