Shapes of Atoms in Collisions

The study of linear and circular polarisation of light induced by inelastic ion-surface collisions has revealed interesting properties of the ion-surface interaction. Application of the technique to single atomic collisions allows investigation under very clean conditions. Coherence and correlation analysis yields in favourable cases a direct visualisation of the shape and dynamics of the electron cloud of the excited atom. The principle of the technique is presented, and some recent examples are selected. The degree to which this information provides new insight into the physical processes governing the excitation is discussed.