High-resolution ion mobility measurements of indium clusters: electron spill-out in metal cluster anions and cations

High-resolution ion mobility measurements have been used to examine Inn− and Inn+ clusters with n=2–30. The cluster–He scattering cross-sections of the anions are systematically larger than those of the cations. The difference (∼25% for In2 to ∼2% for In30) is attributed to the electron density extending further from the surface of the cluster in the anions than in the cations. Geometric cross-sections estimated from electron densities determined from a charged jellium model are in good agreement with the experimental data: the change in cross-section with charge state is reproduced over the entire range.