Femtosecond photoionization of (H2O)(n) and (D2O)(n) clusters

Cluster ion distributions of water in a molecular beam are investigated by femtosecond ionization at 780 nm and reflectron time-of-flight mass spectro metry. The electric field strength generated by the ultrashort laser pulses is sufficient to efficiently ionize most of the molecules that are present in the molecular beam. In this work ion signals of large water clusters co ntaining up to 60 monomers are reported. Upon ionization rapid proton trans fer is observed, leading to the formation of protonated water cluster ions. Unprotonated clusters (H2O)(n)(+) (n>2) are not observed in the mass spect ra. The configurational energy imparted to the protonated clusters induces unimolecular dissociation on the ms time scale. These metastable reactions are characterized by modeling the ion trajectories in the mass spectrometer . The numerical procedure in conjunction with the integrated parent and dau ghter intensities results in unimolecular dissociation rates as a function of cluster size. Additional information about proton transfer reactions is obtained by the investigation of deuterium substitutions. Even though these substitutions correspond to large relative changes in the mass of the atom as well as in the zero point energy, unprotonated (D2O)(n)(+) clusters of significant abundance are not produced in supersonic expansions of deuterat ed water. An additional result of this work is the observation of doubly ch arged ions above a critical cluster size (n=37). (C) 1999 American Institut e of Physics. [S0021-9606(99)01126-5].