The hydrogen fluoride dimer in liquid helium: A prototype system for studying solvent effects on hydrogen bonding

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Abstract
High-resolution infrared spectra are reported for the “free” and “hydrogen bonded” H–F stretches of the hydrogen fluoride dimer solvated in helium nanodroplets. These rotationally resolved spectra provided detailed information concerning the effect of the helium solvent on the vibrational frequencies, rotational constants and tunneling dynamics of the dimer. The end-over-end rotation of the dimer is slowed by a factor of 2.2 by the helium, while the faster rotation about the a axis remains essentially unaffected. The interchange tunneling is reduced significantly (∼40%) in both the ground and vibrationally excited states. The effective tunneling barrier is higher than in the gas phase, making it easier to quench the tunneling motion with a large dc (direct current) electric field. Rapid rotational relaxation is observed from the Ka=1 state, resulting in significant broadening of the corresponding subband. Combination bands are observed for the intermolecular F–F stretch and trans-bend vibrations, providing further insights into the nature of the solvent effects.