The small splitting of atomic energy levels due to relativistic effects and spin-orbit coupling, creating closely spaced spectral lines that appear as a single line at low resolution. This structure provides detailed information about atomic physics and fundamental constants.
The term emerged in early 20th century atomic physics to describe spectral line structure that was 'finer' than previously resolved. It became associated with the fine structure constant (α ≈ 1/137), a fundamental constant governing electromagnetic interactions.
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