Molecular Collision Cross Sections from Infrared Absorption Measurements

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Abstract
Through measurement of the equivalent width (fractional integrated absorption) of collision‐broadened lines of the P branch of the HCl fundamental vibration‐rotation band at 3.5 μ, the Lorentz half‐widths of these lines have been determined and optical collision cross sections deduced therefrom. The cross sections which have been obtained are those for collisions between HCl molecules and Ar, CO2, CO, D2, H2, HBr, He, Ne, N2, O2, as well as other molecules of HCl. One of the more interesting aspects of these cross sections is their dependence on the rotational state of the absorbing molecule and the various forms which this J dependence takes for the different foreign gases. The line widths range between the extremes of 0.233 and 0.0111 cm—1 atmos—1 while the collision cross sections range between 3.04×10—14 and 0.091×10—14 cm2. Also reported are recently measured values of the strengths for the first eight lines of the P branch of the HCl fundamental. The strength of this band based on these measurements has been found to be 143±5 cm—2 atmos—1 at 300°K.