The latest (June) Physics Today includes a reference to a recent paper in Physical Review Letters
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which tidies up the theory of pressure broadening. I've not read the paper itself (which would cost $25) in part because it looks to be too mathematical to do me much good. The American Institute of Physics has published a short review
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Apparently the paper is restricted to interactions between linear molecules - CO2 with itself, oxygen and nitrogen.
In our atmosphere the relatively strong interactions between CO2 and (the bent) H2O molecule are likely to be even more important than the self-interaction between CO2 molecules.
The abstract says that it deals with "collision-induced absorption (CIA) and collision-induced scattering (CIS) bands by gases of centrosymmetric linear molecules." Thus, it is dealing with molecules like carbon dioxide (O=C=O), oxygen (O=O), and nitrogen (N=N), which are significant constituents of the atmosphere, that are linear and that when the atoms are inverted through the center of mass of the molecule gives an identical molecule. In the case of carbon dioxide this means switching the two oxygen atoms. In contrast, the linear triatomic molecule nitrous oxide (N=N=O) when inverted would give (O=N=N) which is distinguishable from the previous orientation both because the two end atoms have been exchanged but also because the central nitrogen atom would be on the opposite side of the center of mass.
The symmetry of such molecules results in them having no permanent dipole moment and restricts their interactions with electromagnetic radiation. The aspect addressed by this paper is that such molecules, when isolated such as in a low pressure gas, cannot absorb the infrared radiation that would excite vibrations that preserve the symmetry of the molecule. For the symmetric diatomic molecules, this means that they have no infrared absorption. For carbon dioxide, this means that there is no infrared absorption by the vibrational mode where the two oxygen atoms move away and towards the central carbon atom at the same time leaving the carbon atom motionless. However, carbon dioxide has two other vibrational modes that do not preserve the centrosymmetric symmetry and do strongly absorb infrared light in certain frequency bands.
At higher gas pressures, such as atmospheric pressure, collisions with other molecules destroy the perfect symmetry of these molecules and result in additional absorption bands in the infrared. When such collision induced absorptions are in the regions of the infrared that are free from other strong absorptions, then they can have a significant effect on the insulation efficiency of the atmosphere against radiative cooling of a planet such as Earth or Venus.
This effect is not what is usually meant by pressure broadening, which refers to collisions broadening the absorption and emission linewidths of transitions that occur in the absence of collisions.
There should be a further effect from collisions that create weakly bound short-lived molecules like carbonic acid (H2CO3) which will have additional vibrational absorption lines with their own rotational fine structure.
Granting the relatively low concentrations of CO2 and H2O in the terrestrial atmospshere, these two molecules will collide with one another a lot less frequently than they collide with oxygen and nitrogen molecules, but since carbonic acid is held together by real - if weak - chemical bonds the collisions should last appreciably longer.
As you say, this does broaden the conventional understanding of "pressure broadening" bu the effect predates the discussion by a few billions years
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