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P. N. Mohammed, P. G. Steffes (Georgia Institute of Technology)
Recently, a model, based on the Van Vleck-Weisskopf lineshape, was developed for the centimeter-wavelength opacity of PH3, which provides an order of magnitude improvement over previous models (Hoffman et al. ICARUS 152, 172-184, 2001) and will be highly useful in the interpretation of the Cassini S-band and X-band radio occultation studies. New laboratory measurements have been conducted which show that this model is also accurate at low pressures and temperatures, and at millimeter wavelengths such as will be employed in Cassini Ka-band radio occultation studies. The opacity of PH3 in a hydrogen/helium (H2/He) atmosphere has been measured at frequencies in the Ka-band region at 32.7 GHz (9.2 mm), 35.6 GHz (8.4 mm), 37.7 GHz (8.0 mm) and 39.9 GHz (7.5 mm) at pressures of 0.5, 1 and 2 bars and at temperatures of 293 K, 209 K and 188 K. Additionally, new high-precision laboratory measurements of the opacity of NH3 in an H2/He atmosphere have been conducted under the same temperature and pressure conditions described for PH3. These new measurements more tightly constrain the NH3 opacity model supporting use of a Ben-Reuven lineshape model. While theory predicts that there should be a smooth transition from the Ben-Reuven lineshape to the Van Vleck-Weisskopf lineshape at low pressures, the data clearly shows that the Van Vleck-Weisskopf lineshape is not appropriate for NH3 under these conditions and that the Ben-Reuven lineshape is preferable over the pressure and temperature range to be encountered by the Cassini Ka-band radio occultation experiments. These measurements will also elucidate the interpretation of millimeter wavelength observations conducted with the NRAO/VLA at 43 GHz (7 mm).
This work is supported by the NASA Planetary Atmospheres Program under grant NAG5-12122.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.