Electron emission from thin foils induced by slowly moving heavy ions

Secondary electron emission is a useful tool to observe electronic interactions of ions penetrating matter. For an 8 nm carbon foil we have measured the well-known Z(i)-oscillations in the electron yield in backward direction with respect to the ion beam. In forward direction we observe an excess yield relative to the backward direction which reflects the shell structure of the projectiles in a similar manner as the mean charge of the emerging ions. This enhanced electron yield in forward direction is probably caused by the capability of the projectiles to accumulate secondary electrons around the moving core. The assumption is supported by the results obtained for the ratio of the forward and backward yields as a function of the projectile energy which is nearly constant in the range below Bohr velocity. (C) 1999 Elsevier Science B.V. All rights reserved.