Ion-molecule reactions and fragmentation patterns in helium nanodroplets.

A study has been made of the ion chemistry of a series of small molecules that have been embedded in helium nanodroplets. In most instances, the molecules H2O, SO2, CO2, CH3OH, C2H5OH, C3H7OH, CH3F, and CH3Cl have been allowed to form clusters, and reactivity within these has been initiated through electron impact ionization. For two of the molecules studied, CF2Cl2 and CF3I, reactivity is believed to originate from single molecules embedded in the droplets. Electron impact on the droplets is thought to first create a helium ion, and formation of molecular ions is then assumed to proceed via a charge hopping mechanism that propagates though the droplet and terminates with charge-transfer to a molecule or cluster. The chemistry exhibited by many of the cluster ions and at least one of the single molecular ions is very different from that observed for the same species in isolation. In most cases, reactivity appears to be dominated by high-energy bond breaking processes as opposed to, in the case of the clusters, ion-molecule reactions. Overall, charge-transfer from He+ does not appear to be a "soft" ionization mechanism.