Determination of the scattering potential for low energy alkali-metal ions from a Mo(001) surface

The scattering potential for ${\mathrm{Li}}^{+}$, ${\mathrm{Na}}^{+}$, and ${\mathrm{K}}^{+}$ ions reflected from Mo(001) has been determined for incident energies of 250, 500, and 2500 eV. This was done by measuring the dependence of scattered intensity as a function of incident polar angle for various total scattering angles for each ion and incident energy. The angle of the sharp cutoff in intensity is compared with the critical angle predicted by the chain model. It is found that at 2500 eV the scattering is adequately described by the Thomas-Fermi-Moliere potential with use of a Firsov screening length reduced by factors of 0.90, 0.95, and 1.0 for ${\mathrm{Li}}^{+}$, ${\mathrm{Na}}^{+}$, and ${\mathrm{K}}^{+}$, respectively. The data are also described by a universal potential proposed by Ziegler, Biersack, and Littmark. Systematic disagreement between computed and experimental cutoff angles exists at low incident energy and low scattering angle. This can be attributed either to a breakdown of the chain approximation or to an additional repulsive term in the scattering potential which is dependent upon the distance from the surface.