Experimental determination of the surface ionisation in electron probe microanalysis

An experimental procedure is described for measuring the surface ionisation values ϕ(o) using thin-film measurements on a wide variety of substrates. The ϕ(o) values are determined by establishing the ratios of the film element X-ray intensities emitted from supported and unsupported thin films for a number of different film thicknesses and extrapolating towards a film thickness approaching zero. A number of 180 ϕ(o) data each for Al Kα and Pd Lα X-radiations between 4 and 30 kV, on substrates ranging from beryllium up to bismuth, have been collected in this way. The purpose of this work was to provide a systematic database on which a variety of existing expressions for ϕ(o) could be tested. In the final assessment procedure, in which we also included 108 data from literature, we compared the performance of the various expressions from literature for ϕ(o), as well as a newly developed one of our own. Our final conclusion is that there is little to be chosen between the three highest-ranking expressions.