In spectroscopy, collision-induced absorption and emission refers to spectral features generated by inelastic collisions of molecules in a gas.
Collision-induced absorption and emission is particularly important in dense gases, such as hydrogen and helium clouds found in astronomical systems.
On first sight, it may seem strange to treat optical transitions of a collisional complex, which may exist just momentarily, for the duration of a fly-by encounter (roughly 10−13 seconds), in much the same way as this was long done for molecules in ordinary spectroscopy.
Molecules thus may be thought of being surrounded by various electric multipolar fields which will polarize any collisional partner momentarily in a fly-by encounter, generating the so-called multipole-induced dipoles.
[6] Collision-induced absorption was first reported in compressed oxygen gas in 1949 by Harry Welsch and associates at frequencies of the fundamental band of the O2 molecule.
[1] Intensities of spectra of individual atoms or molecules typically vary linearly with the numerical gas density.
These exist usually for much longer times than the collisional complexes and, under carefully chosen experimental conditions (low temperature, moderate gas density), their rotovibrational band spectra show "sharp" (or resolvable) lines (Heisenberg uncertainty principle), much like ordinary molecules.
The significance of CIA for astrophysics was recognized early-on, especially where dense atmospheres of mixtures of molecular hydrogen and helium gas exist.
[10][11] The atmospheres of the inner planets and of Saturn's big moon Titan also show significant CIA in the infrared due to concentrations of nitrogen, oxygen, carbon dioxide and other molecular gases.
[12][13][14] However, the total CIA contribution of Earth's major gases, N2 and O2, to the atmosphere's natural greenhouse effect is relatively minor except near the poles.
[17] Instead, nearly the whole infrared is attenuated or missing altogether from the star's emission, owing to CIA in the hydrogen-helium atmospheres surrounding their cores.
[18][19] The impact of CIA on the observed spectral energy distribution is well understood and accurately modeled for most cool white dwarfs.
[21] However, predicting CIA in the atmospheres of the coolest white dwarfs is more challenging,[22] in part because of the formation of many-body collisional complexes.
For example, CIA in the H2 fundamental band, which falls on top of an opacity window between H2O/CH4 or H2O/CO (depending on the temperature), plays an important role in shaping brown dwarf spectra.
M dwarf atmospheres are hotter so that some increased portion of the H2 molecules is in the dissociated state, which weakens CIA by H2--X complexes.
[29][30] Attempts to model the formation of the "first" star from the pure hydrogen and helium gas clouds below about 10,000 K show that the heat generated in the gravitational contraction phase must be somehow radiatively released for further cooling to be possible.