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

Ionization of a sputtered atom implies a minimum energy cost of ‖${\varepsilon }_F$-${\varepsilon }_a^{\infty }$‖ where ${\varepsilon }_F$ is the Fermi energy and ${\varepsilon }_a^{\infty }$ the energy of the atomic orbital involved in charge transfer. When ‖${\varepsilon }_F$-${\varepsilon }_a^{\infty }$‖ 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, ‖${\varepsilon }_F$-${\varepsilon }_a^{\infty }$‖ 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.