Secondary-electron yield as a probe of preequilibrium stopping power of heavy ions colliding with solids

Kinetic emission of secondary electrons (SE) in high-energy ion-solid collisions is strongly correlated to the strength and gradient of the electronic stopping power of the projectile at the entrance and the exit surfaces of the solid. We have measured the total, forward, and backward SE yields of S{sup {ital q}+} (3.9 MeV/u) and C{sup {ital q}+} (1 MeV/u) passing through carbon foils as a function of the initial charge state {ital q} (S, 10--16; C, 2--6) and of the target thickness {rho}{ital x} (3--180 {mu}g/cm{sup 2}). The strong increase of the total and backward SE yields with {ital q} is discussed in the framework of preequilibrium stopping and effective charge of the projectile ion at the entrance of the solid surface. It reveals shell effects of structured projectiles.