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B.P. Michael, J. E. Allen, Jr. (NASA-GSFC )
Conditions in the upper atmospheres of planets are such that the assumption of local thermodynamic equilibrium is no longer valid and the energy level populations for various constituents do not follow a typical Boltzmann distribution. Radiative cooling then becomes extremely important, requiring source functions that are dependent on the collisional deactivation rates between the constituents. Unfortunately, the experimental data available on these deactivation rates are limited even for the most common constituents, resulting in situations where the principal uncertainity in atmospheric models is directly related to the temperature dependence of the collisional relaxation rates of the species involved. To address this need, a low-temperature nonresonant photoacoustic cell has been developed to measure the vibrational-to-translational (V-T) relaxation times of a gas-phase species as a function of temperature over the range from 77 to 300 K. This new cell has been specifically designed to eliminate Helmholtz resonances and includes several improvements over our previous cell enabling us to achieve the desired low temperatures. Characteristics and capabilities of the cell will be presented along with results for V-T relaxation of the {\em v}4 band of CH4 at temperatures below 250 K.
The author(s) of this abstract have provided an email address for comments about the abstract: Patrick.Michael@gsfc.nasa.gov