text: In spectroscopy, collision-induced absorption and emission refers to spectral features generated by inelastic collisions of molecules in a gas. Such inelastic collisions (along with the absorption or emission of photons) 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.