15min:

C. CALLEGARI, I. REINHARD, A. CONJUSTEAU, K. K. LEHMANN AND G. SCOLES, Department of Chemisty, Princeton University, Princeton, NJ 08544.

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 \chemHCN and \chemCH₃CCH 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 10³. This has allowed us to obtain S/N ratios in excess of 10², even in the presence of considerable broadening. Because of the low temperature, only one line could be seen for \chemHCN, while for \chemCH₃CCH at least three lines in both the P and R branches have been resolved. Although Intramolecular Vibrational Relaxation (IVR) is not expected for \chemHCN, and is probably not contributing in the case of \chemCH₃CCH 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.