Azimuthal-angle and energy distributions ofAl2+ejected from Al(100) byAr+bombardment

Azimuthal-angle and energy distributions of ${\mathrm{Al}}^{2+}$ ions ejected from an Al(100) surface by ${\mathrm{Ar}}^{+}$ bombardment have been measured and compared with corresponding measurements for ${\mathrm{Al}}^{+}$. Like the ${\mathrm{Al}}^{+}$ angle distribution, the ${\mathrm{Al}}^{2+}$ azimuthal-angle distribution is generally quite anisotropic and reflects the fourfold symmetry of the Al(100) surface. However, the ${\mathrm{Al}}^{2+}$ angle distribution generally exhibits a much stronger azimuthal anisotropy and a much stronger sensitivity to the kinetic energy of the ejected particles than does the ${\mathrm{Al}}^{+}$ angle distribution. For ${\mathrm{Al}}^{2+}$ kinetic energies below ∼55 eV, the intensity extrema in the ${\mathrm{Al}}^{2+}$ angle distribution occur at the same azimuthal angles as the extrema in the ${\mathrm{Al}}^{+}$ angle distribution. However, the positions of the intensity extrema in the ${\mathrm{Al}}^{2+}$ angle distribution shift by 45° as the kinetic energy of the ${\mathrm{Al}}^{2+}$ ions increases from below 55 to above 55 eV, while the positions of the extrema in the ${\mathrm{Al}}^{+}$ angle distribution are independent of energy. Preliminary molecular-dynamics simulations of the ejection process suggest possible ${\mathrm{Al}}^{2+}$ ejection mechanisms that are in accord with the observed angle and energy distributions. These mechanisms are consistent with the concept that ${\mathrm{Al}}^{2+}$ forms by de-excitation of 2p holes created by energetic Al-Al collisions in the solid.