Critical investigation of the secondary ion emission of pure metals using the pseudoatom method

Abstract

The experimental data of secondary ion emission of 25 pure meta]s were compared to theoretical results calculated using the one dimensional pseudo-atom model. The ionization probabilities were computed at different input data varying within the published experimental limits. Agreement between theoretical and experimental values of the probability of secondary ion emission was mostly better than ah order of magnitude for the metals investigated except for Zn, Ag and Sn.The theoretical description of secondary ion emission from a target due to an energetic primary ion bombardment is rather difficult a problem. The sputtered particles ate mostly neutral, on]y a very small amount of them being in ionized state. The ionization probability is very sensitive to target composition and surface eondition varying in the order of 10 -2-10 -6 range even for pure meta]s according to the experimental results.The different theoretical models [1--6] take into aecount mostly only one possible physical mechanism and they aro rather faz from any realised experimental environment. Even under these eircumstanees they mostly give only some guesses for the order of magnitude and very few of them result in a closed expression which can be evaluated using experimental input data to specify the target materials.The emission of seeondary ions may be caused by different possible mechanisms depencling on the energies of the secondary particles, target material, surface conditions and bombarding ions. In any case, it is a great challenge, even from the computational point of view, to obtain numerical results varying in four orders of magnitude from input data deviating for the different metals only in one order of magnitude. A second question is whether a theoretical model and the ealculated results can be correlated to the experimental ones because of the rather different neglections, assumptions and possible experimental and numerical errors depending on circumstances and the method used.In view of this very intrieate situation we have investigated theoretieal resuhs for 25 pure metals caleulated by a very simple one-dimensional model

Further studies on the secondary ion emission of pure metals using the pseudo-atom method

“…In Table I the results of the new calculations using (10) are shown with the input data given in [3] for 25 metals. Here y+ is the "experimental" value (compiled as described in [3]), R + is the ionisation probability calculated with (1) and R + is the same using (10).…”

“…Here y+ is the "experimental" value (compiled as described in [3]), R + is the ionisation probability calculated with (1) and R + is the same using (10).…”

“…Compiling available experimental resuhs from the literature the agreements and the inherent limits of this simple model were investigated and recently published [3].…”

“…Using the step potential and the pseudo-atom model the ionization probability R + was given [3] as {k I __ k2 12…”