TRIPLE TRANSITIONS IN SOLID HYDROGEN AND DEUTERIUM

The infrared absorption spectrum of the solid hydrogens is well understood in terms of a pairwise multipolar induction mechanism leading to the infrared activity of single and double transitions. However, in recent experiments we have identified various triple transitions for the first $time^{a b}$. There are two basic mechanisms for the simultaneous excitation of three hydrogen molecules upon absorption of one photon of radiation: {\\textbullet} Intensity transfer due to mixing of states. {\\textbullet} Three-body induced dipole moments. The infrared activity of triple transitions due to intensity transfer resulting from mixing of states was suggested by Tipping $et al.^{c}$ As examples of this mechanism we present the transitions $Q_{1}(0) + Q_{1}(0) + S_{0}(0)$ and $S_{1}(0) + Q_{1}(0) + S_{0}(0)$ in both solid hydrogen and deuterium. However, with this concept it is not possible to explain the triple transition $S_{1}(0) + Q_{1}(0) + Q_{1}(0)$ which we have also observed in both isotopic species of the hydrogen crystal. This transition is located at $12788 cm^{-1}$ in solid $para-H_{2}$ and at $9123.5 cm^{-1}$ in solid $ortho-D_{2}$. We present a theory of three-body induced dipole moments in order to account for the linestrength of this very remarkable transition. Experiments to observe the corresponding triple transition in solid HD are currently in preparation.