Secondary-electron yields from thin foils: A possible probe for the electronic stopping power of heavy ions.

Abstract

We have measured heavy-ion-induced (${Z}_{P}$=2,10,18,36,54; 15 keV/u\ensuremath{\le}${E}_{P}$/${M}_{P}$\ensuremath{\le}600 keV/u secondary-electron (SE) yields from sputter-cleaned entrance (${\ensuremath{\gamma}}_{B}$) and exit surfaces (${\ensuremath{\gamma}}_{F}$) of thin solid foils (C, Al, Ti, Ni, and Cu; d\ensuremath{\approxeq}1000 A\r{}) in ultrahigh vacuum (p${=10}^{\mathrm{\ensuremath{-}}7}$ Pa). A pronounced increase of the forward to backward SE yield ratio R=${\ensuremath{\gamma}}_{F}$/${\ensuremath{\gamma}}_{B}$ with increasing ${Z}_{P}$ is observed. The SE yield to energy-loss ratio ${\ensuremath{\Lambda}}^{\mathrm{*}}$=\ensuremath{\gamma}/${S}_{e}$ has been found to be smaller for heavy ions (HI) than for light ions (H and He); i.e., ${\ensuremath{\Lambda}}^{\mathrm{*}}$(HI)${\ensuremath{\Lambda}}^{\mathrm{*}}$(He)${\ensuremath{\Lambda}}^{\mathrm{*}}$(H). Also, at low projectile velocities (${v}_{P}^{2}$50 keV/u), the value of ${\ensuremath{\Lambda}}^{\mathrm{*}}$ increases with decreasing ${v}_{P}$. The velocity and projectile dependence of both R and ${\ensuremath{\Lambda}}^{\mathrm{*}}$ can be described within simple extensions of Schou's SE emission transport theory and a semiempirical Sternglass-type model introduced by Koschar and co-workers as caused by nonequilibrium projectile energy losses ${S}_{e}^{\mathrm{*}}$ near the surfaces. The near-surface energy losses are reduced compared to tabulated bulk energy loss values ${S}_{e}$ both for forward and backward emission under the assumption of a proportionality between SE yields and dE/dx. The ${Z}_{P}$-dependent reduction factors, i.e., the ratios ${S}_{e}^{\mathrm{*}}$/${S}_{e}$, as well as material parameters \ensuremath{\Lambda}=\ensuremath{\gamma}/${S}_{e}^{\mathrm{*}}$, are deduced from the SE yield measurements. Nevertheless, a rough overall proportionality \ensuremath{\gamma}\ensuremath{\sim}dE/dx over four decades of both forward and backward secondary-electron yields \ensuremath{\gamma} and electronic energy losses dE/dx in a wide range of projectile velocities (15 keV/u \ensuremath{\le}${E}_{P}$/${M}_{P}$\ensuremath{\le}16 MeV/u) and projectile nuclear charges ${Z}_{P}$ (1\ensuremath{\le}${Z}_{P}$\ensuremath{\le}92) is found.