Ionization probability of sputtered atoms: Band-structure and collisional effects

Ionization of a sputtered atom implies a minimum energy cost of \ensuremath{\Vert}${\ensuremath{\varepsilon}}_{F}$-${\ensuremath{\varepsilon}}_{a}^{\ensuremath{\infty}}$\ensuremath{\Vert} where ${\ensuremath{\varepsilon}}_{F}$ is the Fermi energy and ${\ensuremath{\varepsilon}}_{a}^{\ensuremath{\infty}}$ the energy of the atomic orbital involved in charge transfer. When \ensuremath{\Vert}${\ensuremath{\varepsilon}}_{F}$-${\ensuremath{\varepsilon}}_{a}^{\ensuremath{\infty}}$\ensuremath{\Vert} is small, as in some experiments with adsorbates, the ionization probability P can be successfully explained within the ``wide-band model.'' In most sputtering situations, however, \ensuremath{\Vert}${\ensuremath{\varepsilon}}_{F}$-${\ensuremath{\varepsilon}}_{a}^{\ensuremath{\infty}}$\ensuremath{\Vert} is large and this model is inadequate. Since P is then small, the most efficient ejection processes have to be considered first. Invoking a combination of band-structure and collisional effects, a model is proposed for ${\mathrm{Cu}}^{+}$ emission from Cu targets, in which the sputtered atom is pushed by another atom, itself interacting with the rest of the solid. Coupling with both s and d states is included. P is calculated for various sets of parameters corresponding to possible physical situations. The results are compared with simple (but approximate) expressions for P recently obtained in a more general theory.

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