ON FTIR SPECTROSCOPY IN ASYNCHRONOUSLY PULSED SUPERSONIC FREE JET EXPANSIONS AND ON THE INTERPRETATION OF STRETCHING SPECTRA OF HF CLUSTERS

A novel experimental technique using an asynchronously pulsed nozzle i n conjunction with an unmodified continuous scan high resolution inter ferometer (BOMEM DA002) is demonstrated for NH3, N2O, CH4 and (HF)(n). Spectral artifacts associated with the pulsed mode of operation cance l rapidly during the ordinary averaging over several scans. The random averaging is analysed using synthetic interferograms. Combined with c ontinuous 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 l ow temperatures. The tentative spectroscopic assignment of (HF)(n) wit h (n = 4, 5, 6) resulting from this investigation and from harmonic ab initio predictions has recently been challenged by a size-selective s econdary beam scattering OPO experiment. We propose an alternative int erpretation of this scattering experiment which leads to agreement wit h our FTIR results and extensive ab initio predictions. The pentamer o f HF is confirmed to be an important species in HF vapor. A key ingred ient in the extended analysis is the assumption of combination bands o f the type V-HF + V-FF along with the V-HF fundamentals. To back our a ssignment, n-dimensional variational grid calculations for the HF stre tching manifold in (HF)(n) (n = 1-5) based on several thousand DZP MP2 ab initio points were carried out. The anharmonic HF stretching funda mental frequency shifts relative to monomeric HF are 19, 29 and 37% la rger than the corresponding harmonic shifts for n = 3, 4, 5. Correctio ns for basis set superposition error and intermolecular zero point bon d weakening effects approximately cancer with these anharmonicity cont ributions, thus explaining the good agreement between harmonic predict ion and experiment. Transition dipole predictions for overtone absorpt ions in HF clusters at the SCF level show a loss of the large intensit y enhancement found and predicted in the fundamental region.