High-temperature H₂- and He-broadening of CO₂ in the ν₁+ν₃ band

H₂- and He-broadening coefficients of CO₂ and their temperature dependence were measured for the R(18) and R(20) transitions of the ν₁+ν₃ band at 3728.4101 and 3729.7122 cm⁻¹, respectively. Measurements were performed using a tunable distributed-feedback laser near 2.7 µm in a high-temperature optical cell. Absorbance spectra were recorded for mixtures of 1–4% CO₂ in H₂ or He at ten pressures between 75 and 600 Torr, covering temperatures of 295–805 K for H₂ and 295–1005 K for He. The upper temperature limit for the H₂ mixtures was imposed by the onset of the reverse water-gas shift reaction. The measured line shapes were fitted with Voigt profiles, with residuals typically less than 1%. Broadening coefficients were obtained from the linear pressure dependence of the fitted collisional widths, and temperature exponents were obtained from power-law fits. The room-temperature H₂-broadening coefficients are 0.1124 and 0.1118 cm⁻¹ atm⁻¹ for the R(18) and R(20) transitions, with temperature exponents of 0.676 and 0.655, respectively. The corresponding He-broadening coefficients are 0.0591 and 0.0583 cm⁻¹ atm⁻¹, with exponents of 0.569 and 0.543. The measured H₂ exponents are 13–16% larger than the constant value adopted in HITRAN 2024 but agree within 6% with recent requantized classical molecular dynamics predictions. These results provide the first H₂- and He-broadening data for the ν₁+ν₃ band and the first experimental validation of He-broadening predictions above 760 K.