The ejection of charged atomic particles from the surface of an Al target during irradiation by 50-keV ${\mathrm{Ar}}^{+}$ ions was studied. At several angles to the incident beam an analysis of emitted ions was performed by electrostatic deflection to a single-particle detector. The spectra show sharp peaks superimposed on a continuum. The peaks correspond to single collisions. From accurate measurements of peak positions, the inelastic energy loss in the Ar on Al collisions is obtained by methods assuming pure single collisions. The values are about 450 eV for the Al recoil peaks at angles from 77° to 50°, and higher values (up to 600 eV) are found for more violent collisions. Variation of the angle which the surface forms to the incident beam did not influence the peak positions. This result supports the single-collision assumption. Absolute measurements of spectral intensities were also performed, and the number of target atoms per ${\mathrm{cm}}^2$ that would account for the measured peak intensities was derived by applying generally accepted theoretical scattering cross sections. Typically, single-collision scattering is obtained from target atoms with density comparable to or less than the density in a {100} crystal plane. Theoretical estimates of multiple scattering are used in a discussion of the peak formation. The target was a single crystal, but the surface included an oxygen impurity, and no lattice-directional effect was observed. The continuum is interpreted; the discussion is based on the absolute intensity measurements and on the theory for penetration of heavy ions in the keV region for random distribution of target atoms.

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