Infrared Line Collisional Parameters of PH 3 in Hydrogen: Measurements with Second-Order Approximation of Perturbation Theory

Room-temperature absorption by PH 3 –H 2 mixtures in the ν 2 and ν 4 bands of phosphine (PH 3 ) has been measured for low pressures. Fits of these spectra are made to determine the width of isolated lines and line mixing in a first-order Rosenkranz approximation. From the previous determinations, we deduce remarks on the lack of accuracy of predicting the collisional process. With the first-order Rosenkranz approximation, the collisional parameters are considered linear with pressure. In this work, we have considered spectra recorded for three doublets: A 1 and A 2 lines in the ν 2 and ν 4 bands of PH 3 diluted with higher H 2 pressure. We show that the line shifts are nonlinear with perturber pressures, which requires testing the fits of the recorded spectra with profiles developed in the secondorder approximation of the perturbation theory. Consequently, the first- and second-order mixing coefficients are determined and discussed. Throughout this study, we also show that the change in the intensity distribution is provided by the population exchange between low energy levels for the two components of doublets A 1 and A 2 lines and is described through the second-order mixing parameter. Thereby, we show the mixing effect on line width.