SECONDARY-ELECTRON YIELDS FROM THIN FOIL - A POSSIBLE PROBE FOR THE ELECTRONIC STOPPING POWER OF HEAVY-IONS

We have measured heavy-ion-induced (ZP=2,10,18,36,54; 15 keV/uEP/MP600 keV/u secondary-electron (SE) yields from sputter-cleaned entrance (B) and exit surfaces (F) of thin solid foils (C, Al, Ti, Ni, and Cu; d1000 A) in ultrahigh vacuum (p=10-7 Pa). A pronounced increase of the forward to backward SE yield ratio R=F/B with increasing ZP is observed. The SE yield to energy-loss ratio *=/Se has been found to be smaller for heavy ions (HI) than for light ions (H and He); i.e., *(HI)<*(He)<*(H). Also, at low projectile velocities (vP2<50 keV/u), the value of * increases with decreasing vP. The velocity and projectile dependence of both R and * can be described within simple extensions of Schous SE emission transport theory and a semiempirical Sternglass-type model introduced by Koschar and co-workers as caused by nonequilibrium projectile energy losses Se* near the surfaces. The near-surface energy losses are reduced compared to tabulated bulk energy loss values Se both for forward and backward emission under the assumption of a proportionality between SE yields and dE/dx. The ZP-dependent reduction factors, i.e., the ratios Se*/Se, as well as material parameters =/Se*, are deduced from the SE yield measurements. Nevertheless, a rough overall proportionality dE/dx over four decades of both forward and backward secondary-electron yields and electronic energy losses dE/dx in a wide range of projectile velocities (15 keV/u EP/MP16 MeV/u) and projectile nuclear charges ZP (1ZP92) is found. © 1990 The American Physical Society.