van der Waals potentials from the infrared spectra of rare gas–HF complexes

Abstract

High-resolution infrared spectra of the Ar–HF, Kr–HF, and Xe–HF van der Waals molecules have been recorded in the vicinity of the H–F stretching fundamentals, ν1, under thermal equilibrium conditions at T≂211 K with a tunable difference-frequency laser. Rotational structure has been observed up to or approaching rotational predissociation, permitting us to model the effective radial van der Waals potentials for these complexes. These potentials provide good estimates for the binding energies, D0, and the van der Waals stretching frequencies, ν3, in the ground (v1=0) and excited (v1=1) states of the molecules. For v1=0 in Ar–HF, Kr–HF, and Xe–HF, we find D0=102, 133, and 181 cm−1 and ν3=39.2, 41.1, and 43.4 cm−1, respectively. The ν3 modes characterized by the model potentials aid in the assignment of the ν1+ν3−ν3 hot bands observed in our spectra. The band centers for the ν1 fundamentals are all down shifted in frequency from the isolated HF monomer by Δν=−9.654, −17.518, and −29.185 cm−1 for the Ar, Kr, and Xe complexes, respectively, indicating that the van der Waals bonds are some 10% to 15% stronger in the excited vibrational state. This increased vibrational attraction also results in a contraction of the van der Waals radial coordinate manifest in the larger rotational constants observed for ν1; ΔB/B0=+0.35%, +1.00%, and +1.75% for Ar–, Kr–, and Xe–HF. We have also observed the Q branch of the ν1+ν2 stretch–bend combination band in Ar–HF some 70.2 cm−1 above the ν1 fundamental with a large negative ΔB/B0=−2.00% implying a strong anisotropy in the potential.