HG Dimer Pair Potential and Vibrational Dynamics: A near-IR Study of All Four Intermolecular Modes
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Date
1995
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Ohio State University
Abstract
High resolution near infrared spectra of the low-frequency intermolecular modes in combination with the high-frequency intramolecular HF stretching modes have been recorded for HF dimer using a slitjet infrared spectrometer. In total, 14 vibration-rotation-tunneling bands are observed allowing all of the 4 intermolecular modes ("van der Waals stretch" $\nu_{4}$, "geared bend" $\nu_{5}$, "out-of-plane torsion" $\nu_{6}$, and "anti-geared bend" $\nu_{3}$) to be probed for the first time. Intensity data on the $\nu_{4}$ and $\nu_{5}$ combinationa bands suggest significant mixing between what has previously been treated as decoupled "van der Waals stretch" and "geared bend" degrees of freedom. Analysis of the HF dimer rovibrational lineshapes reveals mode specific predissociation broadening and a systematic dependence on intramolecular ($\nu_{2}$ "bound" versus $\nu_{1}$ "free" HF) excitation, with significant additional effects due to low-frequency intermolecular excitation. The availability of high resolution data sampling such a range of vibrational energies permits an unprecedented rigorous test of the HF dimer potential surfaces. The observed intermolecular frequencies are in good qualitative agreement with recent full dimensional bound state predictions on the Quack and Suhm pair potential [M. Quack and M. Suhm, J. Chem. Phys. 89, 33 (1992)], though significant discrepancies exist for the highest frequency ("anti -geared bend") mode. Furthermore, though the qualitative trends are correct, recent calculation by von Dirke et al. on the Quack and Suhm surface systematically under predict the influence of intrmolecular excitation on the intermolecular degrees of freedom. This is also consistent with the prediction of predissociation lifetimes that are substantially longer than experimentally observed.
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Author Institution: University of Colorado, Boulder, Colorado 80309-0440.