Only experimentally determined energy levels are included, the energies being restricted to excitations of outer-shell electrons and to inner-shell excitations up to the soft x-ray range. The levels were taken from analyses of the spectra of atomic gases wherever possible; however, the levels for several of the triply ionized rare earths are from analyses of the spectra of the ions in crystals or solutions. In addition to the level value (usually in units of cm 1 ) and the parity, the J value, configuration and term assignments, and the experimental g value are listed wherever available. Leading percentages from the calculated eigenvector are also tabulated for each level if available. The levels are grouped into spectroscopic terms of appropriate coupling schemes where such groups appear meaningful. Ionization potentials are tabulated for most of the spectra. Complete references for the tabulated data are given for each spectrum.
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