Pressure-induced line mixing in the v4 band of NH3 perturbed by O2 and self-perturbed

Pressure-induced line-mixing parameters of 231 rovibrational transitions of several branches in the v(4) band of ammonia perturbed by O-2 have been measured at room temperature (295 K) using a high-resolution Fourier transform spectrometer. The line-mixing parameters were obtained using the method of multi-spectrum fitting to the measured shapes of the lines, including the interference effects caused by the line overlaps. The largest amounts of coupling are observed between the symmetric and asymmetric components of inversion doublets mainly in the P-P branch of the v(4) band. The line-mixing parameters are both positive and negative and are much greater for self-broadening than for O-2-broadening. The estimated error is around 15% and widely depends on the quality of spectral lines. In order to predict these line-mixing parameters, we have calculated state-to-state cross sections for NH3 perturbed by O-2- and self-perturbed using a semiclassical model. The predicted values are in reasonable agreement with the measurements for O-2- and self-broadening. This calculation illustrates that the state-to-state cross section contributions to the line-mixing parameters mainly arises from the energy transfer between symmetric and asymmetric components of the inversion doublets. An examination of these cross sections shows a pronounced J and K quantum number dependence. (C) 2009 Elsevier Inc. All rights reserved.