Resonant low energy electrons and their impact on sampling depth during backscatter-electron ms̈sbauer spectroscopy

Abstract

Collection of low energy electrons (< 15 eV) during Conversion Electron Mossbauer Spectroscopy (CEMS) provides enhanced surface sensitivity. To identify and quantify the potential surface sensitivity provided by these species, spectra were collected from a 92.8% enriched 57Fe foil using retarding field energy analyzers in conjunction with spiraltron electron multipliers. Both resonant and nonresonant count rates decrease by as much as 50% at 10 eV bias potential establishing that a large fraction of low energy electrons is produced during CEMS. Surface enhancement due to low energy electrons was identified by observing inherent signal-to-background ratios from samples with the topmost 1.0 nm chemically labeled. The area ratio of the 1.0 nm overlayer to the substrate was 1.43 at 0 eV bias potential while at 15 eV bias potential the area ratio decreased to 0.72. By vacuum evaporating a 5.0 nm copper coating on the sample, near complete attenuation of the excess low energy electrons from the 1.0 nm overlayer was achieved. These results suggest that some low energy electrons below 15 eV are formed as primary products of electronic relaxation following nuclear decay and that they are not the result of straggling or other scattering phenomena. Low energy resonant electron signals appear useful for surface science applications by permitting information from the topmost monolayers to be readily distinguished from those signals arising from deeper within the solid.