Valence Ionization Of Water Clusters Formed Inside Helium Nanodroplets

The ionization mechanisms of small H$_2$O/D$_2$O clusters embedded in helium nanodroplets (HNDs) irradiated with extreme ultraviolet photons of energy $h\nu$ = 21.6 eV are investigated using Penning ionization electron-ion coincidence spectroscopy. Both protonated (H$_2$O)$_{n-1}$H$^{+}$/(D$_2$O)$_{n-1}$D$^{+}$ ($n$ = 3-6) and unprotonated (H$_2$O)$_{n}$$^{+}$/(D$_2$O)$_{n}$$^{+}$ ($n$ = 2-5) cluster ions were observed. Penning ionization electron spectra measured in coincidence with water cluster ions emitted from water clusters doped into both large and small HNDs are analyzed and compared with photoelectron-photoion coincidence spectra measured for free water clusters at $h\nu$ =20.6 eV. The results reveal suppression of fragmentation inside HNDs and stabilization of intact cluster ions. Quantum chemical calculations support the coexistence of proton-transferred and hemibonded conformers under the cryogenic conditions of helium nanodroplets.