Relaxation matrix for symmetric tops with inversion symmetry: Line coupling and line mixing effects on NH₃ lines in...

Line shape parameters including the half-widths and the off-diagonal elements of the relaxation matrix have been calculated for self-broadened NH₃ lines in the perpendicular ν₄ band. As in the pure rotational and the parallel ν₁ bands, the small inversion splitting in this band causes a complete failure of the isolated line approximation. As a result, one has to use formalisms not relying on this approximation. However, due to differences between parallel and perpendicular bands of NH₃, the applicability of the formalism used in our previous studies of the ν₁ band and other parallel bands must be carefully verified. We have found that, as long as potential models only contain components with K₁ = K₂ = 0, whose matrix elements require the selection rule Δk = 0, the formalism is applicable for the ν₄ band with some minor adjustments. Based on both theoretical considerations and results from numerical calculations, the non-diagonality of the relaxation matrices in all the ^PP, ^RP, ^PQ, ^RQ, ^PR, and ^RR branches is discussed. Theoretically calculated self-broadened half-widths are compared with measurements and the values listed in HITRAN 2012. With respect to line coupling effects, we have compared our calculated intra-doublet off-diagonal elements of the relaxation matrix with reliable measurements carried out in the ^PP branch where the spectral environment is favorable. The agreement is rather good since our results do well reproduce the observed k and j dependences of these elements, thus validating our formalism.