Vibrational spectroscopy of van der Waals bonds: Measurement of the perpendicular bend of ArHCl by intracavity far infrared laser spectroscopy of a supersonic jet

We report the development of a very sensitive laser technique for measuring the vibrational motions in van der Waals bonds. This new technique has been used to detect the lowest perpendicular bending vibration in ArHCl near 35 cm-1 with signal-to-noise ratios > or =104 and with resolution of nuclear hyperfine structure. A total of 80 Stark-hyperfine components of the R(0), Q(1), Q(2), rotational lines have been measured with a precision of < or =1 MHz and analyzed by least squares techniques to obtain precise values for the vibrational band origin [ν0 = 1018 731.20(31) MHz], the dipole moment [μ' = 0.260 26(11) D], the effective rotational constant [(B'+C')/2 = 1696.70(22) MHz], the l-type doubling constant [q/sub l/ = -50.90(35) MHz], and the 35Cl quadrupole coupling constants [eqQ/sup prime//sub a/a = -6.5(4.1) MHz, eqQ/sup prime//sub b/b-eqQ/sup prime/ /sub c/c = -74.4(2.2) MHz] in the excited perpendicular (l = +- 1) bending state. Interpretation of these constants indicates a strongly bent average geometry (theta>510) for the excited state. These measurements strongly support the results obtained from empirical potential surfaces by Hutson, and Howard and Hutson