Knowledge Bank

The characteristics of collision-induced absorption in compressed gases will be reviewed. The induced $\nu 1$ band of gaseous and liquid carbon dioxide shows the molecular frequencies; in the solid, only combination tones of the molecular frequencies and the lattice frequences are present. This result is explained in terms of the crystal structure and the hindered molecular rotation. In solid hydrogen and in the liquid a few degrees above the melting point, the induced fundamental band consists of the superposition of three distinct patterns each of which has Q, S(O) and S(1) lines. One pattern of strong sharp lines and one of weak sharp lines are interpreted as due to the quadrupole interactions of nearest neighbors and more distant neighbors, respectively. The third pattern, of broad lines, is caused by exchange interactions modulated by the lattice frequencies.