Shifting and line mixing parameters in the ν4 band of NH3 perturbed by CO2 and He: Experimental results and theoretical calculations

The pressure-induced shifting coefficients and line mixing parameters have been studied in the ν4 band of NH3 perturbed by CO2 and He at room temperature. Measurements have been made using a high-resolution Fourier transform spectrometer. The measurements cover the PP and RP branches of the ν4 band and are located in the spectral range 1470-1600 cm−1. The line shift and line mixing parameters have been derived from a non-linear least-squares multi-pressure fitting technique. The shift coefficients are compared to a semiclassical calculation based on the Robert-Bonamy formalism employing two types of intermolecular interactions. It is shown that the line shifts mainly originate from the vibrational dephasing effects. The observed interference parameters are compared with calculations based on state-to-state collisional cross sections calculated from the intermolecular potential with a semiclassical approach. The results of computation are in reasonable agreement with the experimental data. It is demonstrated also that the line mixing process mainly originates from the energy transfer between symmetric and antisymmetric components of the inversion doublets.