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Auger electron spectroscopy in coincidence with threshold photoelectrons
Figure 1 shows a schematics of experimental apparatus used for threshold photoelectron-Auger electron coincidence spectroscopy. We have two electron analyzers, for Auger electrons and for threshold electrons. The resolution is practically determined by the photon band width, not by that of Auger analyzer, which is used just to separate the Auger final states.
We are interested in resonant enhancement of double photoionization (DPI) process near an atomic inner shell threshold. In our case, DPI leads to the creation of slow electrons whose emission dynamics is not yet completely elucidated. At the inner shell thresholds, DPI is described in a first approximation as the creation of a zero kinetic energy photoelectron, which we call threshold photoelectron, and an inner shell vacancy followed by Auger decay where a double charged ion and an Auger electron are produced. Due to the post collision interaction (PCI), the interaction between the photoelectron, the Auger electron and the atomic field which changes during the Auger decay, energy exchange between the escaping electrons occurs. Consequently, threshold electrons can be observed, not only at threshold, but also above the inner shell threshold. This phenomenon of PCI distortion of threshold electron yield is well known and has been widely investigated both theoretically and experimentally.However, a detailed understanding of dynamics of slow electron production is still missing.
Figure 1. Experimental apparatus of threshold photoelectron-Auger electron coincidence spectroscopy
Figure 2 shows a threshold photoelectron-Auger electron coincidence spectrum in the vicinity of the Ar2p threshold. In this two dimensional spectrum, the coincidence signals are plotted as a function of the fast electron energy and the photon energy. With the linear relation between the fast electron energy and the photon energy, the final states appear as diagonal lines on the two dimensional spectrum. In this electron energy region, the Ar2+ final states have the electron configuration of (3p4). We can recognize three different channels, corresponding to the 3P, 1D and 1S states. Projecting the coincidence signals in each channel onto the horizontal axis, we obtain the threshold electron spectra for different doubly charged ionic final states.
Figure 2. Auger electron spectrum in coincidence with a threshold photoelectron in the vicinity of the Ar2p ionization thresholds