Photodissociation Studies on (H₂O)_n⁺ (n=2−5) Clusters at 308 nm^†

The photodissociation dynamics of small ( \rmH_2\rmO)_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. (\rmH_2\rmO)_2^+ clusters dissociate to yield \rmH_3\rmO^+ and \rmH_2\rmO^+ photofragments, indicating the presence of both proton-transferred (\rmH_3\rmO^+-OH) and hemibonded (\rmH_2\rmO-\rmO\rmH_2)^+ structures for the dimer cluster. ( \rmH_2\rmO)_n^+ (n=3−5) clusters prevailingly dissociate to the \rmH^+ (\rmH_2\rmO) _n-2,\cdots,1 photofragments by losing both of OH and \rmH_2\rmO components, and the (\rmH_2\rmO)_5^+ cluster shows an additional channel to produce \rmH^+ (\rmH_2\rmO)_4 by only losing OH. The former suggests the (\rmH_2\rmO)_n-2\rmH_3\rmO^+\rmOH structures for the (\rmH_2\rmO)_n^+ (n=3−5) clusters, while the latter suggests in (\rmH_2\rmO)_5^+ that, the \rmH_3\rmO^+ core and OH are separated by \rmH_2\rmO. The results elucidate the structure progresses of small (\rmH_2\rmO)_n^+ clusters. The experimental images yield negative and small values for the anisotropy parameters of photofragments, indicating that (\rmH_2\rmO)_n ^+ (n=2−5) clusters undergo vertical electronic transitions upon photon absorption followed by slow dissociation, and lead to highly internally excited photofragments.