CHARACTERIZATION OFAl+SECONDARY ION EMISSION PRODUCED BYNe+ANDAr+BOMBARDMENT OF ALUMINIUM SURFACE

This paper reports the characterization of the velocity (energy) dependencies of theAl+secondary ion emission produced by 0.5 keV and 5 keVNe+andAr+bombardment of polycrystalline pure aluminium. The distributions of secondaryAl+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=5–25eV)and at a high emission energy(Ek=80–280eV). The linear portions were fit by exponential dependencyP+∝exp(-v0/vn)with two different values of the characteristic velocityv0. For the low emission energy, the valuev01~(3.3±0.2)×106cm/swas 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, namelyv02~(5.3±0.3)×106cm/sforNe+andv02~(8.1±0.3)×106cm/sforAr+; the ratiov02(Ne+)/v02(Ar+)is close to the value$\sqrt{{\bf M}_{\rm Ne}/{\bf M}_{\rm Ar}}$. 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.