The first- and second-order perturbation expressions for the imaginary part of the impact optical cross section for light linear tops at high Js are derived. In this range, the static terms due to V - R coupling are found to be predominant among the contributions to the vibrotational line shift. This opens a new way to deduce data on the vibrational dependence of the intermolecular potential from the experimental shifts of the fundamental and overtone lines. The theory is applied to HCl and HF perturbed by noble-gases and nitrogen. The derived overdetermined system of equations for the characteristics of the intermolecular potential is processed by the linear regression analysis. The results thus obtained are compared with the available potential models. ©2004 Copyright SPIE - The International Society for Optical Engineering.