Cutoff-free theory of impact broadening and shifting in microwave and infrared gas spectra

A cutoff-free impact theory of pressure broadening and shifting of rotation and rotation–vibration spectra in gases is derived. The theory is based on the ATC (Anderson–Tsao–Curnutte) theoretical framework and uses a linked-cluster theorem for degenerate states to obtain a form for the interruption function S(b) in which the dependence on the interaction potential is exponential. S(b) is bounded for 0⩽b⩽∞, allowing integrals over the impact parameter b to be performed without resort to a cutoff. Earlier impact theory results are shown to be limiting cases of our result. Results for broadening cross sections are obtained for certain ideal cases and compared to corresponding ATC results.