Velocity dependence of secondary-ion emission

Abstract

The emission probability ${S}^{+}(v)$ of secondary ions ejected along the surface normal from polycrystalline Cr, Ag, Cu, and Zr is found to be strongly dependent upon the velocity of the secondary ion. The dependence of ${S}^{+}(v)$ on the perpendicular velocity component is in qualitative agreement with theories in which the ion formation is governed by a time-dependent perturbation of the outer electronic structure of the atom as it crosses the boundary of the surface into the vacuum. Plots of ${log}_{10}{S}^{+}(v)$ against ${v}^{\ensuremath{-}1}$ show linearity in the energy region $4\ensuremath{\le}E\ensuremath{\le}30$ eV, with slopes of the order of (2-3)\ifmmode\times\else\texttimes\fi{}${10}^{6}$ cm/sec. Thus exponential dependence of ${S}^{+}(v)$ predicted by the time-dependent perturbation theories is observed, at least over a limited energy range. A simple image-force correction extends adherence to linearity below 4 eV, with ion yields as low as ${10}^{\ensuremath{-}6}$. Positive deviation from the linear dependence of ${S}^{+}(v)$ is observed in each case for secondary ions with $E>30$ eV. This energy region may also be fitted with an exponential dependence of ${S}^{+}(v)$, yielding slopes within the range of (3-6)\ifmmode\times\else\texttimes\fi{}${10}^{6}$ cm/sec.