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Fine structure (spectral lines)
The closely spaced groups of lines observed in the spectra of the lightest elements, notably hydrogen and helium. The components of any one such group are characterized by identical values of the principal quantum number n, but different values of the azimuthal quantum number l and the angular momentum quantum number j.
In atoms having several electrons, this fine structure becomes the multiplet structure resulting from spin-orbit coupling. This gives splittings of the terms and the spectral lines that are “fine’’ for the lightest elements but that are very large, of the order of an electronvolt, for the heavy elements.
(or multiplet splitting), a splitting of the energy levels and spectral lines of atoms, molecules, and crystals that is caused by spin-orbit coupling. The number of sublevels into which an energy level is split depends on the number of possible spin orientations—that is, on the multiplicity K—and does not exceed K. In particular, for alkali-metal atoms the spin of the outer electron can have two orientations (K = 2); in this case, the energy levels are split into two sublevéis (doublet splitting), and the spectral lines are split into two closely spaced levels (doublets). For light atoms the magnitude of the fine-structure splitting of energy levels does not exceed 10–4 electron volt; for spectral lines the corresponding magnitude is several wave number units (cm–1). The splitting increases markedly as the nuclear charge Z becomes larger. The splitting of the energy levels of heavy atoms may reach several tenths of an electron volt.
M. A. EL’IASHEVICH