title: Multiphoton ionization of large water clusters.
abstract: Water clusters are multimers of water molecules held together by hydrogen bonds. In the present work, multiphoton ionization in the UV range coupled with time of flight mass spectrometry has been applied to water clusters with up to 160 molecules in order to obtain information on the electronic states of clusters of different sizes up to dimensions that can approximate the bulk phase. The dependence of ion intensities of water clusters and their metastable fragments produced by laser ionization at 355 nm on laser power density indicates a (3+1)-photon resonance-enhanced multiphoton ionization process. It also explains the large increase of ionization efficiency at 355 nm compared to that at 266 nm. Indeed, it was found, by applying both nanosecond and picosecond laser ionization with the two different UV wavelengths, that no water cluster sequences after n = 9 could be observed at 266 nm, whereas water clusters up to m/z 2000 Th in reflectron mode and m/z 3000 Th in linear mode were detected at 355 nm. The agreement between our findings on clusters of water, especially true in the range with n > 10, and reported data for liquid water supports the hypothesis that clusters above a critical dimension can approximate the liquid phase. It should thus be possible to study clusters just above 10 water molecules, for getting information on the bulk phase structure.
authors.affiliation: Combustion Research Institute, IRC-C.N.R., P.le Tecchio 80, 80125 Napoli, Italy.
authors.forename: B
authors.initials: B
authors.lastname: Apicella
authors.affiliation: Key Laboratory of Power Machinery and Engineering, Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China.
authors.forename: X
authors.initials: X
authors.lastname: Li
authors.affiliation: CNISM and Chemical Engineering, Materials and Industrial Production Department, University of Naples "Federico II," P.le Tecchio 80, 80125 Napoli, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Passaro
authors.affiliation: CNISM and Physics Department, University of Naples "Federico II," Via Cintia, 80124 Napoli, Italy.
authors.forename: N
authors.initials: N
authors.lastname: Spinelli
authors.affiliation: SPIN-C.N.R., Via Cintia, 80124 Napoli, Italy.
authors.forename: X
authors.initials: X
authors.lastname: Wang
mesh_terms: D006860:Hydrogen Bonding; D007477:Ions / Q000737:chemistry*; D007834:Lasers; D044367:Phase Transition; D017785:Photons; D055585:Physical Phenomena; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D014867:Water / Q000737:chemistry*
chemical_list: D007477:Ions; D014867:Water