Self-broadening coefficients in the ν7 band of ethylene at room and low temperatures

Using a tunable diode-laser spectrometer, we have measured self-broadening coefficients for a few transitions in the ν₇ fundamental band of C₂H₄ at 298 and 174 K. The studied transitions J^′, K_a^′, K_c^′←J, K_a, K_c with 3?J?17, 1?K_a?4, and 1?K_c?14 are located in the spectral range 919-982 cm⁻¹. The collisional widths are measured by fitting each spectral line with Voigt, Rautian, and speed-dependent Rautian profiles. The latter model provides larger broadening coefficients than the Rautian profile and still larger coefficients than the Voigt profile. An approximate semiclassical calculation performed by considering only electrostatic interactions leads to reasonable agreement with the experimental data. By comparing the results obtained at room and low temperatures, the temperature dependence of the self-broadening has been determined both experimentally and theoretically.