Rotationally Resolved Infrared Laser Spectroscopy of (H2)n-HF and (D2)n-HF (n = 2−6) in Helium Nanodroplets

Rotationally resolved vibrational spectra are reported for a total of 15 hydrogen-HF clusters formed in helium nanodroplets, each with a different size (up to 6 hydrogens), or isotopic (H 2 /D 2 ) or ortho/para composition. All of these spectra correspond to H−F stretch excitation and are well represented as symmetric or near symmetric prolate tops; fitted rotational constants are presented. Symmetry breaking in the mixed ortho/para complexes gives rise to both parallel and perpendicular transitions in the (H 2 ) n -HF complexes, whereas only parallel transitions are observed in the spectra of the mixed ortho/para (D 2 ) n -HF complexes. This difference, as well as the general trends in the rotational constants, can be understood in terms of the relative zero-point energies of the H 2 and D 2 complexes. Evidence of significant delocalization of the solvating hydrogen molecules is presented for larger pure para hydrogen clusters ( n = 4−6).