Rhenium, tantalum and gold surfaces : an investigation into the origins of peaks in the low energy secondary electron spectrum

Abstract

This thesis reports an investigation into the origins of peaks in the low energy secondary electron spectra of 5d transition metal surfaces. The rhenium spectrum was studied in detail, followed by preliminary studies of tantalum and gold. Spectra were recorded in ultra high vacuum using a cylindrical mirror analyser connected to a microcomputer. An experimental control language interpreter was developed for the data logging. Computer programs were also written for analysis. This included the curve fitting of second derivative experimental spectra with both Lorentzian peaks and Lorentzian peaks broadened by the detecting voltage modulation technique. The program was able to determine peak positions, relative intensities and widths of overlapping peaks from noisy spectra. Such peaks are difficult to resolve by eye. The main peaks in the spectra were due to Auger emission as predicted. However, unexpectedly, peaks were also found which were due to Auger 'cascade' processes and diffraction effects. These overlap the Auger peaks and will distort any measurements taken in quantitative experiments if peaks are assumed to be entirely due to Auger emission. The most intense group of peaks in the spectra (140 to 190eV) were identified as mainly due to N4 5N6 7N6 6,7 super Coster-Kronig transitions, in agreement with current theoretical intensity calculations. However, there is a discrepancy of lOeV between theoretical N4,5N6 7N6 61, line energies and the rhenium experimental peak energies. The discrepancy between experiment and theory is small at each end of the 5d transition metal series and rises to a maximum at rhenium (which has a half full 5d shell). 30% of N4 initial holes were found to decay to an N 5 V final state rather than the N6,7N6,7 final state in rhenium. This compares with the theoretical value of only 1%. The widths of rhenium peaks in this area varied from 5.0eV to 7.6eV which is slightly below the 8eV predicted.

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