Valence ionization of water clusters formed inside helium nanodroplets.

The ionization mechanisms of small H2O/D2O clusters embedded in helium nanodroplets (HNDs) irradiated with extreme ultraviolet photons of energy hν = 21.6 eV are investigated using Penning ionization electron-ion coincidence spectroscopy. Both protonated (H2O)n-1H+/(D2O)n-1D+ (n = 3-6) and unprotonated (H2O)n+/(D2O)n+ (n = 2-5) cluster ions were observed. Penning ionization electron spectra (PIES) measured in coincidence with water cluster ions being emitted from H2O/D2O clusters doped in both large and small HNDs are analyzed, and compared with photoelectron-photoion coincidence spectra measured for free H2O/D2O clusters at hν = 20.6 eV. While the 1b1 outer-valence electron signal associated with direct photoionization of free water clusters suggests that the most abundant cluster ions result from cluster ion fragmentation, the droplet-correlated 1b1 outer-valence electron signal indicates that water cluster ions of various sizes remain largely intact, rather than fragmenting, following their creation by Penning ionization. Quantum chemical calculations on unprotonated clusters confirm the likely coexistence of proton-transferred and hemibonded isomers under the cryogenic environment of HNDs, at least for the trimer and the tetramer.