Theory of relaxation parameters of the spectrum shape in the impact approximation−II: Simplifications, application for qQ(J,K) doublets in the v1 band of NH3 self-broadening

•The physical mechanism of the collision line mixing is discussed.•The line mixing can result in the narrowing of lines which is independent of pressure.•The isolated line approximation is inapplicable for NH3 doublets in the v1 band self-broadening.•Due to line mixing the narrowing of self-broadened doublet components may exceed 20%.

In the first part of this paper the general theory of relaxation parameters of the spectrum shape in the impact approximation was presented. Here we describe some simplifications, discuss the isolated line approximation and compare our results with the other methods of a spectral line half-width and center shift calculation. As an example, illustrative calculations of the qQ(J,K) doublets in the v1 band of NH3 self-broadening have been performed and the results are compared with the experiment.