Knowledge Bank

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 $CH3CCH$ embedded in He clusters $(T=0.4K)$ 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 HCN, while for $CH3CCH$ 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 $CH3CCH$ 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.