We compare the Baranger-type unified theory of line broadening with a quantum version of the binary-collision approach. For the simple model system of a two-state atom, where both treatments are well-defined, the binary-collision theory results only from the exact formalism after an inversion of an integration variable in an integral equation. On the other hand, the binary-collision theory is applicable to more general systems. In the limiting cases of the impact and the quasistatic theories, the two treatments yield identical results
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