Role of projectile electrons in secondary electron emission from solid surfaces under fast-ion bombardment

We report on measurements of the number of electrons per incoming projectile emitted from the projectile entrance and exit surfaces of thin carbon foils as function of the projectile atomic number Z (1 less than or equal to Z less than or equal to 32), incident charge states, and velocities below, near, and above the stopping power maximum. The screening of the projectile charge by the projectile electrons reduces the backward electron emission. The corresponding reduction parameters have been determined as function of the number of projectile electrons. For all ions but protons we note as a general trend that the forward to backward yield ratio R increases with the projectile velocity. In addition a pronounced increase of the R-gamma with increasing Z is also observed. It was found that the increase of the R-gamma with Z in the medium velocity region (similar to 1 MeV/u) is not a ''nuclear charge'' effect, but is simply due to the difference in the incident number of projectile electrons. The experimental results are compared to Monte Carlo simulations based on the work by Gervais and Bouffard. A reasonable agreement (within 20%) between experimental and calculated results for heavy ions is observed. However, the difference is larger for light ions. Finally, the results are discussed within the framework of previously published semiempirical models and the relation between electron yields and the electronic stopping power is elaborated.