Low-energy Cs⁺ scattering from water on Pt(111): A kinetic energy analysis of the Cs⁺-water clusters

We measured the kinetic energy of Cs⁺ and Cs(H₂O)_n⁺ (n = 1-3) cluster ions formed in reactive scattering of low-energy (20-80 eV) Cs⁺ from physisorbed water on Pt(111). Reflected Cs⁺ primaries show two components in the energy spectrum: one scattered from surface water molecules appearing at low energy and another from bare Pt atoms at a higher energy. The former is efficiently converted to Cs(H₂O)_n⁺, the reaction probability being close to unity at full coverage with water. The kinetic energies of the clusters are presented as a function of incident beam energy, scattering angle, and water exposure. On the basis of these results, we propose a cluster formation mechanism in which slow Cs⁺ picks up water molecules via an ion-surface abstraction reaction. At high energy, Cs⁺ penetrates into a physisorbed layer and the clustering reactions may also take place inside the collisionally activated volume of the layer.