Electron emission from carbon foils induced by keV ions

We have measured yields of electrons emitted in the forward and backward directions from ultrathin carbon foils due to 10–100 keV atomic and molecular projectiles. In general, electron yields are higher in the forward than in the backward direction. Their behavior with projectile type and energy can be explained by a competition of a forward peaked angular distribution of initial ionization events and elastic collisions that tend to randomize electron motion in the foil. Experiments with ions with atomic number Z51 – 10 indicate that heavy projectiles produce less electron emission per amount of deposited energy by the projectile near the surface. This is attributed to a larger fraction of low-energy electrons produced by heavy projectiles in the primary ionization event that cannot surmount the surface barrier. For incident molecules, the backward electron yield is less than the sum of the yields of the constituent atoms. Neutral atoms with relatively low ionization potentials produce higher electron yields in the backward direction than the ions of the same species for incident energies above about 5 keV/amu, which is attributed to electron loss from the projectile. @S0163-1829~98!07629-2#