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Abstract:
The gas phase vibrational spectroscopy of Br−⋅(HBr)1,2,3 clusters has been studied between 6 and 16 μm (625 and 1700 cm−1) by multiphoton infrared photodissociation spectroscopy using the output of the free electron laser for infrared experiments. Infrared (IR) spectra were recorded by monitoring the mass-selected ion yield. In all three systems neutral HBr loss is found to be the dominant photofragmentation channel. BrHBr− exhibits a weak absorption band at 1558 cm−1 which is assigned to the overtone of the antisymmetric stretching mode ν3. A series of strong absorption bands was observed for Br−⋅(HBr)2 at energies in the 950–1450 cm−1 range. The Br−⋅(HBr)3 spectra reveal two absorption bands at 884 and 979 cm−1, which are assigned to two H-atom stretching modes. Evidence for the localization of the H atom and destruction of the symmetric BrHBr− hydrogen bond in the larger clusters is presented. Standard electronic structure calculations fail to reproduce the experimental IR spectra, indicating a breakdown of the harmonic approximation.
