Appearance potential spectroscopy

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Abstract
The dependence of total soft x‐ray fluorescence on electron excitation energy is known as appearance potential spectroscopy (APS). The various processes of importance in APS are examined in detail. These include excitation at the threshold for core‐hole production, x‐ray emission during deexcitation, and x‐ray detection by photoelectric emission. The excitation at threshold is controlled by the local density of unfilled states, the position and width (lifetime) of the core‐hole state the appropriate transition probabilities, and the energy spread of the incident electrons. While these parameters completely determine the shape of appearance potential thresholds, they do not completely specify the magnitude. The probability of x‐ray emission (fluorescence yield) also directly influences the magnitude of the thresholds. These various effects lead to strong variations of both the threshold shape and magnitude as a function of Z. Thus while APS is quite sensitive to the 3d transition metals, it is relatively insensitive to semiconductors and insulators and to 4d transition metals.