Hyperthermal surface-collisions of water cluster cations

Size-selected, protonated water cluster cations(H₂O)_nH⁺,4 n 32, are scattered atnormal incidence from the surface of a diamond-coated siliconwafer at collision energies 0 E _coll 500 eV. The size distribution ofcollision-induced fragment-ions and the ion yield of scatteredparticles are analyzed, using a secondary time-of-flight massspectrometer, as a function of the cluster size,n, and the collision energy,E _coll. Even at low impact energies onlyvery small fragment-ions can be detected, with a maximumfragment size of 35% of the colliding parent cluster ions. Forclusters consisting of more than 10 molecules, the protonatedwater dimer(H₂O)₂H⁺becomes the predominant fragment-ion. The total charge survivalyield obeys a nonlinear increase with cluster size; for thelargest clusters investigated, more than 35% of the impactingions survive the surface collision in the cationic charge state.