Photoionization of water clusters at 11.83 eV: Observation of unprotonated cluster ions (H2O)+ n (2≤ n ≤10)

Unprotonated cluster ions (H2O)+n (2≤n≤10) and (Ar)m(H2O)+n (1≤m≤3; 2≤n≤7) are detected in the mass spectra, for the first time, in addition to normally observed protonated ions (H2O)n−1H+ when supersonic cluster beams of water–argon mixtures (P≳2 atm) are photoionized with the vacuum-UV resonance lines at 11.83 and 11.62 eV. The protonated water cluster ions (H2O)n−1H+ are produced by ionization of neutral water clusters (H2O)n followed by intracluster proton-transfer reactions, whereas the unprotonated cluster ions (H2O)+n are generated via the photoionization of water–argon binary clusters: (Ar)m(H2O)n+hν →(H2O)+n+mAr+e−. The excess energies on ionization can be randomized within the [(Ar)m(H2O)+n]*vip clusters (‘‘intracluster excess energy dissipation’’) and finally converted to the decompositions of argon atoms, giving rise to the stable (H2O)+n and various (Ar)k(H2O)+n ions. The (H2O)+n ions have the distinct potential energy minima along the proton transfer reaction coordinates.