Line-mixing effects in He- and N₂-broadened Σ←Δ Q branches of C₂H₂ at low temperatures

Two Q branches of CH near 716.3 and 719.9 cm of ∑ ← ∏ symmetry have been studied for He and N perturbers at pressures ranging from 0.05 to 0.8 atm and low temperatures (-75 and -90°C) using a tunable diode-laser spectrometer. To interpret the strong line-mixing effects observed in these spectra, we have applied a model based on the energy corrected sudden approximation (ECS) whose parameters are derived from those previously determined at room temperature through a fitting procedure of line-broadening coefficients. These coefficients have been measured for a few lines of CH perturbed by He at -100°C. The ECS model provides satisfactory agreement with experimental bandshapes for both bands, temperatures and perturbers at pressures generally higher than 0.1 atm.