Low-pressure spectral line profiles: an analysis in terms of symmetric speed-dependent Voigt profiles

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Abstract
Detailed numerical analysis of profiles of the calcium 422.7 nm resonance line perturbed by rare gases at low densities indicates the inadequacies of the conventional Voigt profile as a description of the lineshapes. Two independent contributions to the line asymmetry are identified: (i) a wing asymmetry whose magnitude is proportional to a typical collision time and which is the first-order correction to the impact approximation; and (ii) a core asymmetry arising from correlations between broadening rate and emitter velocity. An expression, based on classical Fourier analysis and correct velocity averaging, is given for the complete lineshape. This lineshape is found to describe the experimental profiles to a much greater degree of accuracy than the conventional expression, and yields accurate line parameters.