Collisional mixing of (vib)rotational lines appearing in spectroscopic signatures of dense planetary
atmospheres and combustion environments is rigorously handled for the case of two linear colliders
in terms of incomplete (non-Markovian) collisions related to off-energy-shell scattering amplitudes.
Contrary to the standard impact-approximation approaches valid solely in band-centre regions, a
new uniform broadband spectrum description is developed on the basis of a frequency-dependent
rotational relaxation matrix which accurately accounts for the influence of the extra photon energy
with respect to the molecular transitions. This matrix is built using a symmetric Liouville-space
metric and obeys all known fundamental rules. Its direct calculation from refined potential-energy
surfaces and promising modeling methods for forthcoming practical computations are outlined. A
simple preliminary test for N2–N2 isotropic Raman line widths argues in favor of considerable effects
of the internal perturber’s structure on modeled spectral characteristics.