CHARACTERIZATION OF Al+ SECONDARY ION EMISSION PRODUCED BY Ne+ AND Ar+ BOMBARDMENT OF ALUMINIUM SURFACE

Abstract

This paper reports the characterization of the velocity (energy) dependencies of the Al + secondary ion emission produced by 0.5 keV and 5 keV Ne + and Ar + bombardment of polycrystalline pure aluminium. The distributions of secondary Al + ions over their kinetic energy were measured for emission energies of 1–1000 eV without applying electric fields to force the ions into the mass–energy analyzer. To extract the ionization probability, the measured energy distributions of emitted ions were normalized with respect to reference energy distributions of neutral atoms. The reference distributions were obtained by original numerical simulations, as well as analytically, through a sophisticated normalization of the Thompson distribution. It was shown that for both extraction methods, the logarithmic plots of the normalized secondary ion fraction versus the normal component of the reciprocal ion velocity (the reciprocal or inverse velocity plots) are nonmonotonic, with two peaks and two linear portions situated at a low emission energy (Ek=5–25 eV ) and at a high emission energy (Ek=80–280 eV ). The linear portions were fit by exponential dependency P+∝ exp (-v0/vn) with two different values of the characteristic velocity v0. For the low emission energy, the value v01~(3.3±0.2)×106 cm / s was independent of the mass and energy of the projectiles. However, for the high emission energy, the characteristic velocity depended on the projectile's mass, M, namely v02~(5.3±0.3)×106 cm / s for Ne + and v02~(8.1±0.3)×106 cm / s for Ar +; the ratio v02( Ne +)/v02( Ar +) is close to the value [Formula: see text]. This indicates that ballistic mechanisms might contribute to affect the high-energy part of the reciprocal velocity plots along with nonballistic ionization processes, which are generally believed to be the only significant factor for the plots.