Hyperthermal surface-collisions of water cluster cations

Abstract.

Size-selected, protonated water cluster cations (H₂O)_nH⁺, 4 ≤ n ≤ 32, are scattered at normal incidence from the surface of a diamond-coated silicon wafer at collision energies 0 ≤ E _coll ≤ 500 eV. The size distribution of collision-induced fragment-ions and the ion yield of scattered particles are analyzed, using a secondary time-of-flight mass spectrometer, as a function of the cluster size, n, and the collision energy, E _coll. Even at low impact energies only very small fragment-ions can be detected, with a maximum fragment size of ∼35% of the colliding parent cluster ions. For clusters consisting of more than 10 molecules, the protonated water dimer (H₂O)₂H⁺ becomes the predominant fragment-ion. The total charge survival yield obeys a nonlinear increase with cluster size; for the largest clusters investigated, more than 35% of the impacting ions survive the surface collision in the cationic charge state.