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.