MEASUREMENTS OF PURE CO2 ABSORPTION AT HIGH-DENSITIES NEAR 2.3-MU-M

Thermal emission from the deep atmosphere of Venus can be detected on the nightside around 2.3-mu-m. The analysis of this radiation requires a reliable knowledge of the absorption in the far wings of the nearby allowed CO2 bands and of the absorption due to collision-induced bands. We measured absorption coefficients for pure CO2 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 x 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 chi factor extrapolated from the 3800-4000 cm-1 region does not provide enough opacity.