Knowledge Bank

High-resolution spectra of the Rg-HCl van der Waals complexes (Rg = Ne, Ar, Kr) have been recorded in the H-Cl stretching region (near $2900cm-1$) using a tunable difference-frequency laser system. The samples were studied under thermal equilibrium conditions (T = 128 K) at low pressures ($PHCl$ = 2 Torr, $PRg$ = 5-12 Torr) and long path lengths (L = 72-80 m). The $\Sigma -\Sigma$ parallel-type H-Cl stretching fundamentals for the Ar and Kr complexes and the $\Sigma -\Pi$ perpendicular-type stretch-bend combination for all three complexes have been observed. The lower-order ground-state rotational constants agree with prior molecular-beam electronic-resonance results, but the higher J levels available to these infrared measurements probe the entire van der Waals potential surface up to dissociation. Striking rotational predissociation is observed as the rotational energy approaches and exceeds the van der Waals binding energy plus the centrifugal barrier. The vibrational dependence of the intermolecular potentials is manifest in the observed red shifts of the band centers from free HCl and in the excited-state rotational parameters.