Photodissociation studies on (H2O)n+(n=2–5) clusters at 308 nm†.

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. [ABSTRACT FROM AUTHOR]