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[56.04] Measurements of Photoabsorpton Cross Sections and their Temperature Dependence for CO₂ in the 170nm to 200nm Region

W.H. Parkinson, K. Yoshino (CfA)

All the photochemical models for the predominately CO₂ Martian atmosphere ar e very sensitive to the amount of CO₂ and to the values and spectral details of the absorpton cross sections of CO₂ in the region 170nm-200nm. Earlier we had measured and published absolute cross sections of CO₂ in the region 118.0 ~nm-175.5~nm at 295K and 195K.

We have recently extended these measurements from 170~nm to 200~nm at 300K and 1 95K. The new measurements have been carried out at high resolution with our 6.65 -m normal incidence , photoelectric spectrometer. To measure the weak photoabsorption of the CO₂ bands in the wavelength region 170 --200 nm, we required a high column density of the gas. We obtained this by using a multi pass technique, a White cell. The White cell was designed to have a distance of 1.50~m between two main mirrors, and was set for four, double pas ses making a path length of 12.0~m. CO₂ gas was frozen in a stainless cylinder immersed in liquid nitrogen, and t he frozen product (dryice) was pumped by the diffusion pump for purification. The CO₂ was warmed up slowly and kept in the cylinder at high pressure. The CO₂ pressure used in the White cell was varied from 1 to 1000 Torr depend ing on the wavelength region, and was measured with a a capacitance manometer (M KS Baratron, 10 Torr and 1000 Torr). We divided the spectral region into twenty sections of about 1.5 nm extent. At each scan range, another scan was obtained from the emission spectrum of the fourth positive bands of CO for wavelength calibration.

We acknowledge funding from NASA, grant NAGS-7859 to Harvard College Observatory.

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