Evidence for subsurface Auger processes during interactions ofN7+ions with a Ni(110) target

We present secondary-electron spectra arising from collisions of ${\mathrm{N}}^{7+}$ ions with a Ni(110) target for kinetic energies ranging between 150 eV and 20 keV and also for different observation angles. The filling of the two K-shell holes in ${\mathrm{N}}^{7+}$ requires two consecutive KLL Auger processes, giving rise to a double structure in the KLL peak. The ratio of the intensities of the two KLL peaks depends on the collision energy as well as on the angle of observation. We show that this dependence can be explained by assuming that the KLL electrons are emitted after the projectile has penetrated the surface.