DIODE-LASER MEASUREMENTS OF COLLISIONAL LINE BROADENING IN THE UPSILON-5 BAND OF C2H2

Self-broadening coefficients for 27 lines of acetylene in the P- and R-branches of the v5 band have been measured at room temperature with a tunable diode-laser spectrometer. We have first calculated these broadening coefficients on the basis of two semi-classical impact models by using a simple intermolecular potential. The quadrupole moment of C2H2 as well as a dispersion parameter have then been determined by fitting the calculated results of our experimental data. We next use the approach of Robert and Bonamy in which the intermolecular potential does not include adjustable parameters and involves the addition of an atom-atom interaction model to the electrostatic interactions. The usual spherical harmonics expansion of the atom-atom potential appears to be quite inadequate at short distances. The results of calculations derived from a rough consideration of the exact radial expansion of the atom-atom potential lead to better agreement for high J lines than those derived from the truncated expansion. It is also shown that the electrostatic contributions arising from the hexadecapole moment of C2H2 are only significant for \m\ > 20.