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Quantification of the surface composition of low-work function systems using low-energy noble gas ion scattering is strongly influenced by resonant neutralization. Below a certain threshold in the work function, resonant neutralization to the first excited level of the ion is possible and the neutralization probability of the ions increases exponentially with decreasing work function. We show how to correct for the influence of the work function and perform the quantification using the characteristic velocity method. The discussion and conclusions here are relevant for all ion scattering studies using low-energy noble gas ions on surfaces with a work function below ∼4 eV.

The work function dependence of the neutralization of low-energy He ϩ , Ne ϩ , and Ar ϩ ions was studied by determining the neutralization probability of ions scattered from submonolayer coverages of Ba on W͑110͒ and Re͑0001͒ substrates. At high work functions ͑Ͼ3.5 eV͒, it was found that the dominant neutralization mechanism for noble gas ions with initial energy between 2 and 5 keV scattering from Ba is collision-induced neutralization. The neutralization probability for this mechanism was found to be insensitive to work function changes. We argue that collision-induced neutralization is also the dominant charge transfer process for scattering from other earth-alkali and alkali elements in this energy range, although at lower energies it is expected that Auger neutralization will become important. At work functions below roughly 3.5 eV, resonant neutralization to the first excited level of the noble gas ions occurs in addition to the charge transfer processes operating at high work functions. We show that the additional neutralization at low work functions can be described using resonant charge exchange theory. Due to resonant neutralization, the neutralization probability for noble gas ions increases exponentially with decreasing work function.

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The signal intensity in low-energy ion scattering depends, among others, on the kinetic energy of the scattered ions. The measured energy dependence is a combination of the intrinsic energy dependence, determined by the ion-sample interaction, and the energy dependence of the instrumental response function. The intrinsic energy dependence is often exploited to obtain information on the charge transfer processes taking place between the ions and the sample surface. The resulting information is of relevance both for quantitative surface analysis as well as for obtaining a fundamental understanding of charge transfer processes. For a correct determination of the intrinsic energy dependence it is crucial that the energy dependencies of the analyser transmission and the ion detection efficiency of the instrument are accurately known.In this paper we outline how these energy dependencies may be obtained, and we present the results for the cylindrical mirror analyser and microchannelplate-based detector in our apparatus. The discussions about the energy dependencies are not specific for our apparatus, but can be applied directly to other cylindrical mirror analysers and channelplate-based detectors. Therefore, the similarities and differences with other types of analysers are briefly discussed. The implications of uncertainties in these energy dependencies for charge-exchange studies and quantitative compositional surface analysis are treated in detail.