Vibrational dependence of line coupling and line mixing in self-broadened parallel bands of NH3

Abstract Line coupling and line mixing effects have been calculated for several self-broadened NH 3 lines in parallel bands involving an excited ν 2 mode. It is well known that once the ν 2 mode is excited, the inversion splitting quickly increases as this quantum number increases. In the present study, we have shown that the ν 2 dependence of the inversion splitting plays a dominant role in the calculated line-shape parameters. For the ν 2 band with a 36 cm −1 splitting, the intra-doublet couplings practically disappear and for the 2ν 2 and 2ν 2 - ν 2 bands with much higher splitting values, they are completely absent. With respect to the inter-doublet coupling, it becomes the most efficient coupling mechanism for the ν 2 band, but it is also completely absent for bands with higher ν 2 quantum numbers. Because line mixing is caused by line coupling, the above conclusions on line coupling are also applicable for line mixing. Concerning the check of our calculated line mixing effects, while the present formalism has well explained the line mixing signatures observed in the ν 1 band, there are large discrepancies between the measured Rosenkranz mixing parameters and our calculated results for the ν 2 and 2ν 2 bands. In order to clarify these discrepancies, we propose to make some new measurements. In addition, we have calculated self-broadened half-widths in the ν 2 and 2ν 2 bands and made comparisons with several measurements and with the values listed in HITRAN 2012. In general, the agreements with measurements are very good. In contrast, the agreement with HITRAN 2012 is poor, indicating that the empirical formula used to predict the HITRAN 2012 data has to be updated.