Secondary electron emission enhancement by metal monolayer adsorption

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Abstract
Changes in secondary electron emission (SEE) during absorption of a metal vapour can be followed by measuring the changes in the net current icc flowing into the specimen crystal. Plots of icc as a function of deposition time t (for a fixed primary beam energy) are presented for five different adsorbate-substrate systems (Pb or Sn on different Cu single crystals). Comparison is made with results from Auger spectroscopy. Adsorption up to the complete monolayer coverage usually causes significant enhancement of emission. Sharp breaks in the icc-t plots permit an accurate identification of different stages of adsorption. The plots are particularly sensitive to disorder of the adlayer. It is suggested that the observed effects are not due only to work function changes but that there are electron scattering events at the adsorbate-substrate interface which can enhance ionisation in the surface region. Such interfacial effects may be important for quantitative surface electron spectroscopies. The acronym SEECC is proposed for this simple method for surface characterisation.