A level described by the removal of one electron from the configuration of the neutral
ground state. These levels form a spectrum similar to that of a one-electron or hydrogen-like
atom but, being single-valency levels, have the energy scale reversed relative to
that of single-electron levels. Diagram levels may be divided into
valence levels and core levels according to the nature of the electron vacancy. Diagram levels
with orbital
angular momentum different from zero occur in pairs and form spin doublets.
Source:
PAC, 1991, 63, 735
(Nomenclature, symbols, units and their usage in spectrochemical analysis - VIII.
Nomenclature system for X-ray spectroscopy (Recommendations 1991))
on page 738