Measurements of Photoabsorption Cross Sections for H2O in the 170 nm to 200 nm Region

Details Measurements of Photoabsorption Cross Sections for H2O in the 170 nm to 200 nm Region Measurements of Photoabsorption Cross Sections for H2O in the 170 nm to 200 nm Region

Sep 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002dps....34.3701p&link_type=abstract

American Astronomical Society, DPS Meeting #34, #37.01; Bulletin of the American Astronomical Society, Vol. 34, p.907

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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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