Vibrational spectroscopy of small (HF)n clusters (n=4–8) in size-selected molecular beams

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Abstract
Molecular beam depletion and fragment spectroscopy has been employed to study the absorption behavior of small hydrogen fluoride clusters [(HF)n, n=4–8] in the spectral region between 3100 and 3700 cm−1. As tunable infrared radiation source we used an IR‐seeded optical parametric oscillator (OPO) with a LiNbO3 crystal as nonlinear medium. Size‐specific information has been obtained by scattering the cluster beam from a secondary rare gas beam. In this way all spectral features are assigned to specific cluster sizes. While the qualitative agreement with earlier experiments is good, the new assignments differ by one or more monomer units. In general, it is found that the absorption bands must be assigned to larger clusters. For some cluster sizes a doublet structure is observed. This observation is discussed in terms of combination bands involving intra‐ and intermolecular modes; but the possibility of isomeric structures is considered, too.