Auger photoelectron coincidence spectroscopy studies: Trends in the L_2,3-M_4,5M_4,5 line shapes across the 3d transition-metal series

The Auger-emission process from solids can be very complex, with a variety of processes producing intensity in the final spectra. The L_2,3-M_4,5M_4,5 (L_2,3-MM) Auger spectra of the 3d transition metals, in particular, show many complicating effects. Auger photoelectron coincidence spectroscopy (APECS) is a technique that can be used to elucidate the source of this complexity. In the APECS experiment, Auger electrons are recorded only when the corresponding electron from the ionization event is also detected. In this way only those features that result from the particular initial ionization event in question are measured, thereby greatly reducing the complexity in the Auger spectra. Although a number of successful APECS studies have been made of the emission processes observed in the Auger lines of several individual 3d transition metals, none have yet studied trends across the series. In order to study some of these trends we have made the most systematic APECS study to date of the L_2,3-MM Auger spectra of the metals Fe, Co, Ni, and Ga (in GaAs). The L_2,3-MM lines were observed in coincidence with the 2p_3/2 and 2p_1/2 photoemission electrons in all of these materials. From these studies and the Cu coincidence (APECS) results of Haak and Sawatzky a number of trends are clear and these are reported and discussed with the view of aiding quantitative Auger electron spectroscopy. It has become clear that there is also a need for a consistent and clear nomenclature to describe and distinguish between the various processes which can lead to a common multiple-hole final state. A nomenclature is proposed that aids a clearer comparison and discussion of the contributions of the various processes to the total observed Auger line shape.