Density expansions for the autocorrelation function of spectral-line profiles
Gerhard C. Hegerfeldt, Henrik Schulze · Physical Review A · 1989
We derive two expansions in powers of the density for the autocorrelation function of a spectral-line profile in a plasma. The expansions can be regarded as systematic generalizations of the well-known unified theory and of the scalar theory which appear in a natural way as the respective first-order terms. Under the additional assumption of completeness of collisions, but without assuming nonoverlapping, we obtain a simplified expansion. It is a generalization of the impact theory, and the latter results again from the first-order term. We present quick and direct derivations of the first- and second-order terms for all three density expansions. The Nth-order terms contain time-development operators and S matrices for N or fewer particles.