Journal of Chemical Physics, Vol.117, No.14, 6493-6499, 2002
Gas phase infrared spectroscopy of cluster anions as a function of size: The effect of solvation on hydrogen-bonding in Br-center dot(HBr)(1,2,3) clusters
The gas phase vibrational spectroscopy of Br-.(HBr)(1,2,3) clusters has been studied between 6 and 16 mum (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 nu(3). 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.