Gas phase infrared spectroscopy for cluster anions as a function of size: The effect of solvation on hydrogen-bonding in Br~-·(HBr)_(1,2,3)Clusters

摘要

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 esperiments. 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 v_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.