Abstract
Electron impact mass spectra have been recorded for helium nanodroplets containing water clusters. In addition to identification of both H(+)(H(2)O)(n) and (H(2)O)(n)(+) ions in the gas phase, additional peaks are observed which are assigned to He(H(2)O)(n)(+) clusters for up to n=27. No clusters are detected with more than one helium atom attached. The interpretation of these findings is that quenching of (H(2)O)(n)(+) by the surrounding helium can cool the cluster to the point where not only is fragmentation to H(+)(H(2)O)(m) (where m < or = n-1) avoided, but also, in some cases, a helium atom can remain attached to the cluster ion as it escapes into the gas phase. Ab initio calculations suggest that the first step after ionization is the rapid formation of distinct H(3)O(+) and OH units within the (H(2)O)(n)(+) cluster. To explain the formation and survival of He(H(2)O)(n)(+) clusters through to detection, the H(3)O(+) is assumed to be located at the surface of the cluster with a dangling O-H bond to which a single helium atom can attach via a charge-induced dipole interaction. This study suggests that, like H(+)(H(2)O)(n) ions, the preferential location for the positive charge in large (H(2)O)(n)(+) clusters is on the surface rather than as a solvated ion in the interior of the cluster.
Highlights
Protonated water clusters, H+͑H2On, have aroused intense scientific interest in recent years
Water clusters ranging in size from 1 to 30 molecules could be observed in the mass spectra
Ter ions are H+͑H2On, which are the dominant cationic species seen in the electron impact ionization ofH2On clusters in the gas phase
Summary
Protonated water clusters, H+͑H2On, have aroused intense scientific interest in recent years. These ions are known to be important in the chemistry of the upper atmosphere, but more fundamentally, they serve as model systems for gaining insight into the solvation structures and dynamics of protons in bulk aqueous solutionssee, for example, Ref. 2͒. Vertical ionization deposits a relatively large amount of energy into the cation which is sufficient to induce cluster reorganization and which results in the loss of OH. This process has recently been modeled using ab initio molecular dynamicsAIMD.. This process has recently been modeled using ab initio molecular dynamicsAIMD. ForH2On+ clusters with up to six water molecules, the simulations indicate a two-step process in which ionization first induces a proton to move along a hydrogen bond towards the adjacent water molecule, resulting in the
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