THE INFRARED-ABSORPTION SPECTRUM OF CARBON-DIOXIDE ICE FROM 1.8 TO 333 MU-M

The optical constants of solid carbon dioxide are needed, for example, to accurately calculate the energy balance of the seasonal polar caps of Mars, which are composed primarily of CO2 ice, Only a few measurem ents of uncertain accuracy have been made in the broad wavelength regi ons of weak absorption between the isolated narrow infrared absorption bands of solid CO2, which determine the reflectance and emissivity of these polar caps, A laboratory experiment was therefore undertaken to improve on and extend the wavelength range of the previous data by me asuring transmission through thick samples of high quality to determin e the spectral absorption coefficient alpha in infrared wavelength reg ions of low absorption between 1.8 and 333 mu m (30-5555 cm(-1) in wav enumber). A technique was developed to grow clear, thick CO2 ice sampl es from the gas at a temperature of 150 K, with optical path lengths b etween 1.6 and 107.5 mm. The extremely fine spectral structure of this material was measured using resolutions finer than 0.5 cm(-1). The tr ansmission data for all thicknesses are combined in a minimizing routi ne that estimates both the scattering losses for each sample (typicall y <20% extinction) and alpha (between similar to 0.1 to 4000 m(-1)) at each wavelength, on the assumption that the scattering varies slowly with wavelength, The accuracy of the derived absorption coefficients w as also estimated and is best in regions where alpha is not near the l imits of measurement. Absorption coefficients obtained from the new me asurements are an order of magnitude or more lower than earlier estima tes in the transparent wavelength regions between the strong infrared bands. Also, many weak bands between 1.8 and 5.2 mu m, which were only poorly defined previously, have been more accurately measured.