Collisional interference of lines in the spectra of symmetric top molecules: I. A theory of relaxation parameters of the shape of the spectrum in the impact approximation

The peculiarities of broadening of spectral lines of symmetric top molecules that are related to the double degeneracy of JK levels at K ≠ 0 are considered. It is shown that, irrespective of the potential barrier height for the inversion vibration, transitions between such levels should be considered as doublets whose components are coupled with each other by the collision-induced spectral exchange. The shape of the doublet is described by the well-known Ben-Reuven formula, and, in this case of molecules with a high potential barrier for the inversion vibration, is a dispersion contour with a half-width equal to the difference between the halfwidth of the component of the doublet and the cross-relaxation parameter.