text: Intensities of spectra of individual atoms or molecules typically vary linearly 
  with the numerical gas density. However, if gas densities are sufficiently 
  increased, quite generally contributions may also be observed that vary as density
  squared, cubed... These are the collision-induced spectra of two-body (and 
  quite possibly three-body,...) collisional complexes. The collision-induced spectra
  have sometimes been separated from the continua of individual atoms and 
  molecules, based on the characteristic density dependences. In other words, a 
  virial expansion in terms of powers of the numerical gas density is often 
  observable, just as this is widely known for the virial expansion of the equation 
  of state of compressed gases. The first term of the expansion, which is linear 
  in density, represents the ideal gas (or "ordinary) spectra where these 
  exist. (This first term vanishes for the infrared inactive gases,) And
  the quadratic, cubic,... terms of the virial expansions arise from optical 
  transitions of binary, ternary,... intermolecular complexes, which are 
  (often unjustifyably) neglected in the ideal gas approximation of spectroscopy.