TRIPLE TRANSITIONS IN SOLID HYDROGEN AND DEUTERIUM

$^{a}$ M. Mengel, B.P. Winnewisser, and M. Winnewisser, J. Mol. Spectrosc., in press, (1998). $^{b}$ M. Mengel, B.P. Winnewisser, and M. Winnewisser, J. Low Temp. Phys. (Proceedings of the Cryocrystal '97 Conference), in press, (1998). $^{c}$ R.H. Tipping, Q. Ma, and J.D. Poll, Phys, Rev. B 22, 12314-12317 (1991).Author Institution: Physikalisch-Chemisches Institut, Justus-Liebig-Universit\""{a}tThe 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