Metastable Evaporation\nof Molecules from Water Clusters

We probe the stability of water clusters by means of\ntheir metastable\ndecay probability extracted from two-dimensional reflectron time-of-flight\nmass spectra. Two different methods are used to ionize and potentially\nexcite the clusters and trigger the evaporation: (i) attachment of\nelectrons with near-zero energies, producing negatively charged (H2O)n− clusters, and (ii) electron impact ionization,\nproducing protonated (H<sub>2</sub>O)<sub><i>n</i></sub>H<sup>+</sup> clusters. The electron attachment is a soft ionization\nand therefore provides information about the size distribution of\nthe neutral clusters in the beam due to a very limited amount of post-ionization\nloss of water molecules. A dependence of metastable fractions on the\nconditions of neutral clusters production prior to the electron attachment\nis reported. For the cations, the higher energy electron impact ionization\nleads to a more extensive metastable loss of water molecules. The\nresults are discussed in the light of neutral cluster excitation energy\ndistributions and, for negative clusters, also in terms of binding\nenergies. The experiments demonstrate clearly the role of the excess\nelectron vs the excess proton in the two different charge states of\nthe clusters around sizes <i>N</i> = 50–55, for which\nbinding energies of the anions are derived from the data.