Conservation of resonance frequency in the collisional transitions between spectral lines

Collisional transitions between molecular lines give rise to relevant modifications of the spectral distribution of absorption. We discuss here the consistency of such a coupling effect with the conservation of resonance frequency for the scattered lines, a requirement that, in the frame of the usually adopted impact models, we prove to follow from the conservation of energy in the collisions. This problem, considered here for the first time, imposes a new approach to the well known collisional coupling effect and could affect heavily the lineshape in the far wings. A check allowing better insights into this question could be obtained by measurement of the microwave absorption of deuterated ammonia.