Slit jet infrared spectroscopy of NeHF complexes: Internal rotor and J-dependent predissociation dynamics

Direct absorption tunable difference frequency IR spectroscopy in a slit jet supersonic expansion has been used to observe complexes of Ne with HF for the first time. Spectra of both the weak HF stretch fundamental (1000)←(0000) and the 10–20 fold more intense bend and stretch combination band (111e,f0)←(0000) transitions are observed, and illustrate several interesting dynamical features. The large ratio of combination band to fundamental intensity is evidence for a highly isotropic potential with respect to HF rotation. The HF bend vibration is thus better thought of as nearly free internal rotor motion with a nearly good space fixed quantum number, jHF =1. Weak anisotropy in the potential permits the jHF=1 (Πebend) levels to relax intramolecularly to jHF =0 (Σ) levels, leading to predissociative line broadening in the sub-Doppler slit jet spectra. This observed dissociation of NeHF with 44 cm−1 of internal excitation provides a rigorous upper limit to the binding energy. The Πfbend levels, on the other hand, have no lower energy internal rotor states of the correct parity to relax into, and thus the (111f0) ←(0000) spectra exhibit linewidths limited by the apparatus resolution. The internal relaxation of the jHF=1 (Πebend) levels results from Coriolis mixing with the Σbend states (1200) by overall rotation of the NeHF complex, and thus the lifetimes depend dramatically on J. The J-dependent predissociation rates observed are in remarkably quantitative agreement with quantum scattering calculations on the CEPA ab initio surface of ONeil and co-workers.