On FTIR Spectroscopy in Asynchronously Pulsed Supersonic Free Jet Expansions and on the Interpretation of Stretching Spectra of HF Clusters

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Abstract
A novel experimental technique using an asynchronously pulsed nozzle in conjunction with an unmodified continuous scan high resolution interferometer (BOMEM DA002) is demonstrated for NH3, N2O, CH4 and (HF)n. Spectral artifacts associated with the pulsed mode of operation cancel rapidly during the ordinary averaging over several scans. The random averaging is analysed using synthetic interferograms.Combined with continuous jet FTIR spectroscopy, the new technique allows a wide range of expansion conditions to be realized. This has proven to be useful in the investigation of HF stretching spectra of HF clusters down to low temperatures. The tentative spectroscopic assignment of (HF)n with (n = 4, 5, 6) resulting from this investigation and from harmonic ab initio predictions has recently been challenged by a size‐selective secondary beam scattering OPO experiment. We propose an alternative interpretation of this scattering experiment which leads to agreement with our FTIR results and extensive ab initio predictions. The pentamer of HF is confirmed to be an important species in HF vapor. A key ingredient in the extended analysis is the assumption of combination bands of the type vHF + vFF along with the vHF fundamentals. To back our assignment, n‐dimensional variational grid calculations for the HF stretching manifold in (HF)n (n = 1–5) based on several thousand DZP MP2 ab initio points were carried out. The anharmonic HF stretching fundamental frequency shifts relative to monomeric HF are 19, 29 and 37% larger than the corresponding harmonic shifts for n = 3, 4, 5. Corrections for basis set superposition error and intermolecular zero point bond weakening effects approximately cancel with these anharmonicity contributions, thus explaining the good agreement between harmonic prediction and experiment. Transition dipole predictions for overtone absorptions in HF clusters at the SCF level show a loss of the large intensity enhancement found and predicted in the fundamental region.