text: Ordinary spectroscopy is concerned with the spectra of single atoms or molecules. Here we outline the very different spectra of complexes consisting of two or more interacting atoms or molecules: the "interaction-induced" or "collision-induced" spectroscopy. Both ordinary and collision-induced spectra may be observed in emission and absorption and require an electric or magnetic multipole moment - in most cases an electric dipole moment - to exist for an optical transition to take place from an initial to a final quantum state of a molecule or a molecular complex. (For brevity of expression we will use here the term "molecule" interchangeably for atoms as well as molecules). A complex of interacting molecules may consist of two or more molecules in a collisional encounter, or else of a weakly bound van der Waals molecule. 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 seconds), in much the same way as this was long done for molecules in ordinary spectroscopy. But even transient complexes of molecules may be viewed as a new, "supermolecular" system which is subject to the same spectroscopic rules as ordinary molecules. Ordinary molecules may be viewed as complexes of atoms that have new and possibly quite different spectroscopic properties than the individual atoms the molecule consists of, when the atoms are not bound together as a molecule (or are not "interacting"). Similarly, complexes of interacting molecules may (and usually do) acquire new optical properties, which often are absent in the non-interacting, well separated individual molecules.
  Collision-induced absorption (CIA) and emission (CIE) spectra are well known in the microwave and infrared regions of the electromagnetic spectrum, but they occur in special cases also in the visible and near ultraviolet regions. Collision-induced spectra have been observed in nearly all dense gases, and also in many liquids and solids. CIA and CIE are due to the intermolecular interactions, which generate electric dipole moments. We note that an analogous collision-induced light scattering 
  (CILS) or Raman process also exists, which is well studied and is in many ways completely analogous to CIA and CIE. CILS arises from interaction-induced polarizability increments of molecular complexes; the excess polarizability of a complex, relative the sum of polarizabilities of the noninteracting molecules.