Collisional reaction of water cluster cations (H2O)n+ (n=2 and 3) with D2O

Collisional reactions of size- and energy-selected unprotonated water clusters (H2O)n+ (n=2 and 3) in a single-collision with D2O were investigated at very low collision energy ranging from 0.05 to 2.0 eV using guided-ion beam tandem mass spectrometry. First, in the unimolecular and collision-induced dissociations of (H2O)2+ and (H2O)3+, only the H2O+ fragment, but not the H3O+ fragment, was observed. These results suggest that the structure of (H2O)2+ is primarily hemibonded ‘(H2O⋯OH2)+’ rather than proton-transferred ‘(H3O+⋯OH)’. Second, the measured incorporation cross-section of (H2O)2+ was found to be smaller than that of H+(H2O)2. From the dipole moment obtained by ab initio calculation, we found that the small values of the incorporation cross-section for (H2O)2+ were caused by the small value, 0.0 debye, of the dipole moment of the cluster.