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 mum (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 nu(3) and a sequence of nu(3)+nnu(1) combinations (n=1-3). Additional features to the blue of these bands spaced by similar to21 cm(-1) are attributed to combination bands involving motion of the Ar messenger atom. Differences in the relative intensities of the nu(3)+nnu(1) combinations bands in comparison to previous matrix experiments are rationalized on the basis of the underlying dissociation dynamics. (C) 2003 American Institute of Physics.