Collision operators for Stark line shapes: When does the unified theory fail?

It is shown that the unified theory for Stark broadening leads to incorrect results when correlated emitter–perturber collisions are present. With a kinetic theory treatment, we propose an extension of the model able to account for such correlations. The treatment presents analogies with renormalization techniques and the resulting collision operator has a structure similar to that obtained within the unified theory. We illustrate the applicability of the model through calculations of hydrogen line shapes in ideal cases.