High resolution 1.3 μm overtone spectroscopy of HF dimer in a slit jet: K a=0←0 and K a=1←0 subbands of vacc=2←0

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Abstract
Continuous wave difference frequency mixing of a single mode Nd:YAG laser at 1.06 μm and a scanning, single mode ring dye laser (R6G) in a LiNbO3 crystal generates a novel source of widely tunable near infrared radiation in the 1.2–2.2 μm region. In conjunction with the high sensitivity of a pulsed slit nozzle expansion with multipass optics (0.48 m path length), this narrow band source of tunable ir light allows the high resolution study of overtone (v=2←0) spectra for a wide variety of molecular complexes with H stretching vibrations. In this paper, we report the first rotationally resolved spectra of (HF)2 in the first HF stretching overtone region. In particular, we observe Ka=1←0 and 0←0 subbands for a vibrational state from one member of the v=2 overtone triad in (HF)2 with a band center of 7682.8228(5) cm−1. We tentatively assign this state as the hydrogen bond acceptor (i.e., free) HF stretching overtone 2νacc based on predissociation line widths and excellent agreement with predictions based on an anharmonic local mode description of (HF)2. Splittings of 0.2119(5) cm−1 (K’a = 0) and 0.0942(3) cm−1 (K’a = 1) due to interconversion tunneling are found. From the observed intensity alternation due to nuclear spin statistical weights, the overall vibrational symmetry for K’a = 0 and 1 is unambiguously determined to be Γvib=A+ and B+ for the lower and upper tunneling levels, respectively. These A+ and B+ symmetry designations correspond to irreducible representations of the MS4 molecular symmetry group, which allows for large amplitude motion and exchange of the identical HF subunits. Predissociation line broadening is observed in each of the four upper vibrational levels which varies between 56(20) and 175(25) MHz and depends sensitively on both K’a and the tunneling symmetry. This tunneling symmetry dependence, together with the unusual K’a dependence of the tunneling splitting and the anomalously large intensity ratio between the parallel and perpendicular transitions, indicates the presence of vibrational resonances in the overtone region not clearly evidenced in the analysis of the corresponding fundamental HF stretch region. Our results are discussed in the context of earlier static cell FTIR spectra and recent ab initio predictions for this overtone state. The data suggest that the overtone dynamics in (HF)2 can not be satisfactorily described as an oscillator pair connected by a 1D interconversion pathway, and may instead involve substantial coupling to other intermolecular vibrational degrees of freedom.