Intracavity Far Infrared Laser Spectroscopy of Supersonic Jets: Direct Measurement of the Vibrational Motions in van der Waals Bonds

Molecules held together by van der Waals forces generally posess bond strengths which are -0 1-1.0% of normal chemical bonds, i.e. 0.1-1.0 Kcal/mole (.35-350 cmi ). Such extremely weak bonds exhi~it vibrational frequencies which lie in the far-infrared, eq. 10-100 cm(1000-100 11m). In order to measure vibration-rotation spectra of van der Waals bonds, one must devise a method which possesses very high sensitivity in this region of the spectrum, and which preferably also has very high resolution capabilities, such that the hyperfine interactions, which can provide important information on the potential surface of these clusters, can be resolved. In this paper we report the development of such a new spectroscopic technique for directly measuring vibrational absorption spectra of van der Waals bonds. In this method, van der Waals molecules are produced in a supersonic free jet expansion located inside the optical cavity of an optically pumped far infrared laser. Electric field tuning of dipole-allowed vibration-rotation transitions into coincidence with the laser frequency produces an extremely sensitive detection method which also possesses very high (-1 MHz) resolution. We shall describe this new technique and its initial applicati~n for the measurement of the bending (\12) vibration in ArHCl near 34 cm•