Cutoff-Free Theory of Impact Broadening and Shifting of Microwave and Infrared Molecular Spectra

Abstract

The semiclassical Anderson-Tsao-Curnutte (ATC) impact theory [1,2] of pressure broadening in molecular spectra has been successful in calculations of linewidths and shifts in the microwave and infrared regimes. However, the theory suffers from a fundamental defect in that, for small impact parameter, b, the interruption function, S(b), attains unphysically large values. In an attempt to circumvent this difficulty, Murphy and Boggs [3] developed an alternative method that used an exponential expansion of the collision matrix elements; this expansion led to finite values of the matrix elements for all impact parameters. Cattani [4] grafted the Murphy-Boggs expansion onto the ATC theory; a similar result was later derived by Salesky and Korff [5]. These latter three approaches neglect the important reorientation terms that were shown by Gordon [6] to be significant.