N₂- and O₂-broadening coefficients of C₂H₂ IR lines

Pressure-broadening parameters of six lines belonging to the ν band of CH in collision with N have been measured with a tunable diode-laser spectrometer in order to complete up to J = 33 our earlier measurements (D. Lambot, G. Blanquet, and J. P. Bouanich, J. Mol. Spectrosc.136, 86-92 (1989)) on the broadening of CH by N and O at 297 K. These N- and O-broadening coefficients have been first calculated on the basis of the Anderson-Tsao-Curnutte theory; in this approach, we show that the short-range interactions which contribute significantly to the linewidths are not correctly treated. Next, we consider the improved semiclassical model proposed by Robert and Bonamy. The intermolecular potential consists in the addition of the atom-atom interaction model to the quadrupolar interactions. The limited radial spherical harmonics expansion of the atom-atom potential, from which expressions for the differential cross section were derived, appears to be quite insufficient at short intermolecular distances. Therefore, we use a more accurate representation of this potential, avoiding an inadequate truncation and keeping the analytic expressions obtained by Bonamy and Robert. In the calculations we take into account the contributions derived from the radial functions U(r), U(r), and U(r), as well as from U(r). A theoretical expression is obtained for the U contribution to the differential cross section. The results of the calculations arising from the exact radial expansion of the atom-atom potential appear to be significantly larger for high J lines than those arising from the truncated expansion. The latter results, which do not include adjustable atom-atom parameters, are in good agreement with experimental broadening coefficients for CHO and in reasonable agreement (except at large J values) for CHN. It is also shown that the contributions to the linewidths derived from U are rather small for CHN and more important for CHO. Finally, by calculating the collisional linewidths of CHN and CHO at 200 K, we have predicted their temperature dependences.