Sub-Doppler infrared spectroscopy in slit supersonic jets. A study of all three van der Waals modes in ? 1-excited ArHCl

Abstract

Direct absorption tunable difference frequency i.r. laser spectroscopy has been used to study the vibrational dynamics of ArHCl complexes cooled in a slit supersonic jet expansion. As a result of large-amplitude vibrational motion in these weakly bound complexes, transitions to each of the three van der Waals modes are observed as combination bands built on the fundamental HCl stretch, thus permitting detailed study of low-frequency, intermolecular modes with a near-infrared laser source. Our measurements of the (11 10)Π(‘perpendicular’) bend state under uncongested supersonic jet (10 K) conditions dictate a complete reassignment of the vibrational band, which resolves a recent controversy between near- and far-i.r. studies. The bathochromically shifted (1200)Σ(‘parallel’) bend provides strong support for the presence of a secondary minimum in the potential at the inverted ArClH configuration. The unexpectedly large strength of combination band transitions to the (1001)Σ van der Waals stretch state suggests the importance of bend–stretch coupling in the potential as a mechanism for stealing intensity from the bends. This is further substantiated by Coriolis analysis of the rotational levels, in which strong J-dependent mixing of all three vibrations is observed. The molecular constants are in excellent agreement with both far-i.r. experimental results and semiempirical predictions for the Ar + HCl (v= 0) surface, indicating only a small change in the intermolecular potential upon vibrational excitation of the HCl.