Probing a strong hydrogen bond with infrared spectroscopy: Vibrational predissociation of BrHBr–·Ar

The gas phase vibrational spectroscopy of BrHBr–, a prototypical strongly hydrogen-bonded species, has been studied between 6 and 17 µm (590 and 1670 cm–1) by infrared vibrational predissociation of the BrHBr–·Ar ion. Infrared excitation was accomplished using the output of the free electron laser for infrared experiments (FELIX). Predissociation spectra were recorded by monitoring depletion of mass-selected BrHBr–·Ar ions as a function of excitation wavelength. Four prominent absorption bands are observed at 733, 890, 1048, and 1201 cm–1. They are assigned to the fundamental of the hydrogenic stretching mode 3 and a sequence of 3 + n1 combinations (n = 1–3). Additional features to the blue of these bands spaced by ~21 cm–1 are attributed to combination bands involving motion of the Ar messenger atom. Differences in the relative intensities of the 3 + n1 combinations bands in comparison to previous matrix experiments are rationalized on the basis of the underlying dissociation dynamics