On the extension of the Rydberg-Ritz principle to the collisional relaxation of molecular and atomic lines

As shown by recent experimental results, the frequently used additivity rules for the collisional broadening and shift are not exact since the Rydberg-Ritz combination principle cannot be extended to the relaxation-modified frequencies of atomic and molecular spectroscopic transitions. We present a treatment allowing the identification of additive and nonadditive parts of the relaxation. We conclude that even in cases when additivity approximately holds, this does not mean that the relaxation of the lines can be reduced to the relaxation of their initial and final states.