Measurements of pure CO₂ absorption at high densities near 2.3 μm

Thermal emission from the deep atmosphere of Venus can be detected on the nightside around 2.3 μm. The analysis of this radiation requires a reliable knowledge of the absorption in the far wings of the nearby allowed CO 2 bands and of the absorption due to collision‐induced bands. We measured absorption coefficients for pure CO 2 at pressures varying from 30 to 60 bars in the frequency range 3910–4570 cm −1 at 297.5 K. Values between 1.0 and 1.6 × 10 −7 cm −1 amagat −2 are found in the 4100–4500 cm −1 interval where emission from the Venus nightside occurs. The comparison of experimental results with synthetic spectra calculated from a line by line code demonstrates that the Lorentzian line shape strongly overestimates the observed absorption, whereas the use of a χ factor extrapolated from the 3800–4000 cm −1 region does not provide enough opacity.