General expansion for the elastic cross-section in the theory of pressure-induced shifts of infrared molecular lines

Expressions for the j-dependent pressure-shift are derived by applying Anderson's theory in the general elastic case. It is shown, that the non-commuting part of the interaction energy gives no contribution to the cross-section up to the third order term included. Numerical values of the shifts are given for the pure rotation and the vibration-rotation lines of the HCl molecule perturbed by Argon. The j-dependence of the shifts is fairly well represented by the theory. The calculation does not apply to the R0 and P1 lines. For this particular case, a formula based upon the sudden approximation has been derived. This approximation is then compared to the adiabatic one used by Ben-Reuven in a previous calculation.