Affiliation:
1. Hefei National Research Center for Physical Sciences at the Microscale, and Department of Chemical Physics, University of Science and Technology of China , Hefei 230026, China
Abstract
The photodissociation dynamics of small (H2O)n+ (n=2–5) clusters have been studied at 308 nm using a high resolution cryogenic cylindrical ion trap velocity map imaging spectrometer. Time-of-flight mass spectra and images of ionic photofragments are recorded. (H2O)2+ clusters dissociate to yield H3O+ and H2O+ photofragments, indicating the presence of both proton-transferred (H3O+-OH) and hemibonded (H2O-OH2)+ structures for the dimer cluster. (H2O)n+ (n=3–5) clusters prevailingly dissociate to the H+(H2O)n–2, …,1 photofragments by losing both of OH and H2O components, and the (H2O)5+ cluster shows an additional channel to produce H+(H2O)4 by only losing OH. The former suggests the (H2O)n–2H3O+OH structures for the (H2O)n+ (n=3–5) clusters, while the latter suggests in (H2O)5+ that, the H3O+ core and OH are separated by H2O. The results elucidate the structure progresses of small (H2O)n+ clusters. The experimental images yield negative and small values for the anisotropy parameters of photofragments, indicating that (H2O)n+ (n=2–5) clusters undergo vertical electronic transitions upon photon absorption followed by slow dissociation, and lead to highly internally excited photofragments.