Sputtering of indium using Aum projectiles: Transition from linear cascade to spike regime

We have investigated the yields and emission velocity distributions of neutral In atoms and In₂ dimers sputtered from a pure indium surface under bombardment with Auₘ^-(m=1,2,3) projectile ions. It is shown that 5-keV Au₁ bombardment leads to results in full compliance with linear cascade sputtering theory. All polyatomic projectiles are found to generate an additional low-energy contribution to the sputtered flux, which increases with increasing projectile nuclearity and energy and completely dominates the spectra under 10-keV Au₃ bombardment. Analysis shows that this contribution cannot be explained in terms of thermal spike sputtering models. Instead, the results indicate a spike emission mechanism, which closely resembles a free expansion of a supercritically heated subsurface volume.