Collision cross sections of small water clusters -: art. no. 023203

The effective integral collision cross section of small water cluster molecules, [H2O](N), with N from 4 to 8, has been measured. A controlled mixture of water vapor and argon, expanding into vacuum, produces neutral water clusters in a supersonic molecular beam. The clusters undergo collisions in a scattering cell with target aases Ar, He, or N-2, and are detected by a quadrupole mass spectrometer after electron-impact ionization. The detected cluster ions are in the form of proton hydrates H+[H2O](K). The apparent cross sections determined from attenuation measurements are approximately of 3 x 10(-18) m(2) for Ar and N-2 target gases, and 10(-18) m(2) for He. Additional data, with a beam having a smaller proportion of heavy clusters gives the same cross section for attenuation of the K = 3 cluster, which suggests that there one dominant channel for the generation of ions from clusters.