Diatomic molecules and general considerations
A Nitrogen molecule N2 in isolation does not interact to any significant extent with infrared light; one might think that collisions do not change this picture, as N2 has no lines to be broadened by collisions. Nonetheless, during the time a collision is taking place the pair of colliding molecules momentarily behaves somewhat like a more complex four-atom molecule, which has transitions that can indeed absorb and emit infrared radiation. This leads to collision-induced absorption, whose associated absorption coefficient is generally a smooth function of wavenumber. Because of the lack of line structure, such absorption is referred to as a continuum. There are many possible processes through which collisions can induce absorption. The collision can impart a temporary dipole moment to a rotation or vibration that ordinarily had none, allowing it to absorb or emit a photon. The collision can break a symmetry, allowing transitions that are otherwise "forbidden" by symmetry principles. Colliding molecules can form dimers, which are short-lived complexes which nevertheless persist long enough to have radiatively active transitions not present in the colliding molecules. Most of the "non-absorbing" diatomic molecules, including N2 and H2, exhibit significant collision-induced continuum absorption at sufficiently high densities. These are mostly associated with the induced-dipole mechanism, and therefore can to some extent be anticipated on the basis of the underlying transitions of the diatomic molecule.
Collision-induced absorption can be thought of as a trinary chemical reaction involving the two colliding molecules and a photon. The rate of "reaction" (i.e. absorption) is proportional to the product of the concentrations of the two colliding species with the photon concentration, the latter being proportional to the radiation flux. The absorption coefficient is the rate constant for the reaction. Unlike the case of collision- broadened line absorption, in collision-induced absorption there is no physical distinction between the "absorbing" molecule and the "perturbing" molecule. Both are equal partners in the process allowing absorption or emission of a photon. For this reason, it is most natural to describe collision-induced absorption in terms of a binary absorption coefficient, which expresses the proportionality between the product of the concentrations of the two colliding species and the rate of absorption of radiation. Nonetheless, in order to facilitate comparison with the previously defined line absorption coefficients, and in order to make it easier to incorporate collisional continuum absorption in radiation calculations which also take into account line absorption, it is convenient to characterize the collision-induced absorption by mass-specific absorption coefficients in which one of the colliding molecules is arbitrarily designated the "absorber," whose absorption is enhanced in proportion to the partial pressure of the "collider". For example, for an N2-H2 collision in a box of gas with uniform temperature T and uniform N2 partial pressure p, the optical thickness can be expressed as
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