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In spectroscopy, collision-induced absorption and emission refers to spectral features generated by inelastic collisions of molecules in a gas. Such inelastic collisions may induce quantum transitions in the molecules, or the molecules may form transient supramolecular complexes with spectral features different from the underlying molecules. Collision-induced absorption and emission is particularly important in dense gases, such as hydrogen and helium clouds found in astronomical systems.
Collision-induced absorption and emission is distinguished from collisional broadening in spectroscopy in that collisional broadening comes from elastic collisions of molecules, whereas collision-induced absorption and emission is an inherently inelastic process.