Upper left: plot of the redistribution function defined in Eq. (61), as a function of the reduced frequency of the incoming photons (x′). Each curve corresponds to a different (integer) value of the reduced frequency of the outgoing photon (x). For some curves, the value of x is indicated in the plot. The reduced frequencies x and x′ are defined with respect to the frequency ν₀ corresponding to the energy separation between the centers of gravity of the two terms (x = (ν₀ − ν)/Δν_D, x′ = (ν₀ − ν′)/Δν_D). We consider a ²S − ²P transition, the two fine-structure components having a separation of 15 Doppler widths (the energies of the upper levels are calculated according to the L − S coupling scheme). We assume an Einstein coefficient A_uℓ = 10⁸ s^-1, a Doppler width Δλ_D = 60 mÅ, and we accordingly calculate the damping constant as a = A_uℓ/(4πΔν_D) ≈ 10^-3 (collisional broadening is neglected, and the conversion of the Doppler width from wavelength units to frequency units is made assuming that the transitions fall at 5000 Å). Upper right: same as upper left, but considering only a few values of x (indicated in the plot). Lower left: same as upper left, but for the redistribution function. Lower right: same as lower left, but considering only a few values of x (indicated in the plot). The inset panel is a zoom of the line core region of the 1/2–1/2 transition, showing in more details the curves obtained for values of x close to x = 10 (the frequency position of the 1/2–1/2 transition).