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[37.01] Measurements of Photoabsorption Cross Sections for H₂O in the 170~nm to 200~nm Region

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

All of the photochemical models of the Martian atmosphere are very sensitive to the amounts of CO₂ and to the values of the absorption cross sections of CO₂ and H₂O around 200~nm. We have recently extended the measurements of CO₂ in the region 170~nm - 200~nm at 295K and 195K. We have now also measured the photoabsorption cross section of H₂O from 180~nm to 198~nm at 295K. These new measurements are extensions of our earlier published photoabsorption cross section of H₂O at 295K in the wavelength region 125~nm to 185~nm.

The new measurements have been carried out with our 6.66-m normalincidence, photoelectric spectrometer. To measure the weak photoabsorption of the H₂O in the wavelength above 190~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 one double pass making a path length of 3.0~m. For the wavelength below 190~nm, we used a short absorption cell located between the spectrometer and the light source (hydrogen discharge). Water was frozen at liquid nitrogen temperature, and was pumped by the diffusion pump for purification. The water pressure used in the absorption cell was varied from 0.1 to 17 Torr depending on the wavelength region, and was measured with a a capacitance manometer (MKS Baratron, 10 Torr and 100 Torr). We divided the spectral region into seven sections of about 3 nm extent. We also measured the abs! ! olute cross sections of water at twelve wavelengths, and these values are used to calibrate the scanned cross sections. 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 NAG5-7859 to Harvard College Observatory.

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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.