Theory of Optical Absorption by Diatomic Molecules Embedded in Rare Gas Crystals

A new mechanism for the study of lattice dynamics is presented, rotation‐translation coupling. With this mechanism the optically active rotational modes of diatomic molecules are coupled to the lattice vibrations of crystals in which the diatomic molecules are embedded. Using recently derived expressions which incorporate force constant changes in the vicinity of the defect we calculate and identify localized modes seen in the IR spectrum of HCl and HBr in rare gas crystals. Further we show how the lattice vibrations shift and broaden the rotational levels of the molecule. It is thought that this mechanism will prove useful in the study of the density of states of pure and impure crystals.