Velocity dependence of secondary-ion emission

The emission probability $S^{+}\mathrm{}\left(v\right)\mathrm{}$ 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 $S^{+}\mathrm{}\left(v\right)\mathrm{}$ 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 ${log}_{10}{S}^{+}\mathrm{}\left(v\right)\mathrm{}$ against $v^{-1\mathrm{}}$ show linearity in the energy region $4\le E\le 30$ eV, with slopes of the order of (2-3)×${10}^6$ cm/sec. Thus exponential dependence of $S^{+}\mathrm{}\left(v\right)\mathrm{}$ 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 ${10}^{-6\mathrm{}}$. Positive deviation from the linear dependence of $S^{+}\mathrm{}\left(v\right)\mathrm{}$ is observed in each case for secondary ions with $E>\mathrm{}30\mathrm{}$ eV. This energy region may also be fitted with an exponential dependence of $S^{+}\mathrm{}\left(v\right)\mathrm{}$, yielding slopes within the range of (3-6)×${10}^6$ cm/sec.