1. General characteristics of X-ray emission spectra

1.1 Introduction: origin and position of spectral lines

X-ray emission lines result, like optical lines, from transitions of atomic electrons between two energy levels; but here at least one of these two levels is a deep level; the X-ray lines corresponding to a given element are far fewer in number than optical lines, so they are easier to identify; however, their relative observable intensities obey laws that are not always simple, and it is not possible to designate a priori and with certainty ultimate lines as in optical spectra (cf. the article Theory of atomic spectra