Transport theory for convoy electrons and Rydberg electrons in solids

Recent experimental investigations of excitation of near-threshold states in fast (vp much-gt 1; vp: projectile velocity) highly charged (q much-gt 1, q: ionic charge) ions traversing thin solid targets have revealed several interesting and heretofore unexplained features. For convoy electrons, i.e. electrons in low-energy continuum states in the projectile frame just above threshold, anomalous transport properties as well as a high degree of anisotropy in the angular distribution have been reported. Convoy electrons give rise to a cusp shaped peak in the forward emission spectrum of electrons whose velocity vector rvec v matches the projectile velocity, rvec v = vrvecp. For high Rydberg states just below threshold an abundant population of high ell states, absent in ion-atom collisions under otherwise identical conditions of projectile velocity and charge state, has been observed. A quantitative description of the evolution of the atomic charge cloud around fast highly charged ions in solids within the framework of classical transport theory is being developed. This amounts to the solution of a phase space master equation for the distribution function f(rvec r,rvec v) which takes properly into account competing effects of the strong Coulomb field, of screening, and of the stochastic perturbations in the solid due to multiple scattering. A numerical solution can be achieved by solving the associated Langevin equation for individual phase space trajectories and employing Monte Carlo sampling techniques. In this progress report the author briefly outlines the theoretical framework and presents first results on the transport effects of Coulomb focussing, defocussing and trapping. 6 figs