THE AR-HF INTERMOLECULAR POTENTIAL - OVERTONE SPECTROSCOPY AND AB-INITIO CALCULATIONS

The vibrational dependence of the intermolecular potential of Ar-HF is investigated through the spectra of levels correlating with HF(v = 3) . We have previously reported measurements of the (vbKn) = (3000), (31 00), and (3110) levels of Ar-HF using intracavity laser-induced fluore scence in a slit supersonic jet [J. Chem. Phys. 98, 2497 (1993)]. Thes e levels are found to be well reproduced (within 0.1 cm-1) by the Ar-H F H6(4,3,2) potential [J. Chem. Phys. 96, 6752 (1992)]. The second ove rtone experiments are extended to include the (3002) state which is co upled to (31 10) through Coriolis interaction, and the (3210) state wh ich is more sensitive to higher-order anisotropic terms in the potenti al. The observations establish that the level (3002) lies 0.229 cm-1 b elow (3110), with upper state rotational constant B = 0.085 89 cm-1. T his is in good accord with the predictions of the H6(4,3,2) potential. The (3210) state lies at 11 484.745 cm-1 with B = 0.099 79 cm-1. The band origin is 1.7 cm-1 higher than predicted, and thus contains impor tant new information on the vibrational dependence of the potential. S everal detailed features of the spectra can be explained using the H6( 4,3,2) potential. The Q-branch lines of the (3210) - (0000) band show evidence of a weak perturbation, which can be explained in terms of mi xing with the (3112) state. The (3210) spectrum exhibits parity-depend ent rotational predissociation and the widths of the P- and R-branch l ines and the magnitude of the 1-type doubling can be explained in term s of coupling to the (3200) state, which is estimated to lie 4 cm-1 be low the (3210) state. The Q-branch lines show a predissociation cutoff above Q (16); this is in reasonable agreement with the predictions of the H6(4,3,2) potential, but suggests that the binding energy calcula ted for the potential may be about 1 cm-1 too large. To examine the po tential further, high-level ab initio calculations are performed, with an efficient basis set incorporating bond functions. The calculations give a well depth of 92%-95% of that of the H6(4,3,2) potential at th eta = 0-degrees for v = 0 and v = 3, respectively; this is in line wit h earlier results on rare gas pairs. The calculations also reproduce t he anisotropy of the H6(4,3,2) potential and its vibrational dependenc e. The dependence of the intermolecular potential on HF bond length is found explicitly.