Surface impact ionization of polar-molecule clusters through pickup of alkali atoms

The observation that clusters of neutral H2O (refs 1-4) or SO2 (ref. 5) molecules, on impact with essentially any solid surface, can decay efficiently into positively and negatively charged fragments has defied explanation, not least because the kinetic energy per molecule can be much smaller than the molecular ionization potentials. Here we present a microscopic model of the charging mechanism, based on a mass analysis of charged SO2 cluster fragments, which appears to be applicable to polar-molecule clusters more generally. Our mass spectra reveal that all positively charged fragments carry an alkali ion (sodium, potassium or caesium), whereas the negative fragments are simply (SO2)(n)(-). The yields of both charged species are comparable, and can be enhanced significantly by pre-treating the sample surface with additional alkali atoms. The key to charge separation in the clusters therefore appears to be the pickup of a neutral (but readily ionized) adatom during impact, followed by delocalization of the adatom's valence electron within the cluster and the subsequent collision-induced fragmentation of the cluster into charged pieces. This process could be of practical use in, for example, charge-pair generation and surface analysis; it may also be relevant to atmospheric ionization processes.