Cold reactions of alkali-metal and water clusters inside helium nanodroplets.

The reaction of alkali-metal (Na, Cs) clusters with water clusters embedded in helium nanodroplets is studied using femtosecond photoionization as well as electron-impact ionization. Unlike Na clusters, Cs clusters are found to completely react with water in spite of the ultracold helium droplet environment. Mass spectra of the Csn+(H2O)m reaction products are interpreted in terms of stability with respect to fragmentation using high-level molecular structure calculations.