Knowledge Bank

A rich spectrum composed of hundreds of transitions with typical linewidth of $\sim 20$ MHz has recently been $observed1$ in the fundamental Q branch of solid p-$H2$ (containing $\sim$ 0.2% o-$H2$) using high resolution difference frequency infrared laser spectroscopy. These features have been ascribed to the anisotropic electrostatic quadrupole-quadrupole and crystal field splitting of the vibrational transition of a pair of interacting $o-H2$ molecules. While we are far from good understanding of the rich spectrum, attempts have been made to assign the transitions due to nn pairs of $o-H2$, based on group theoretical considerations, and the observed frequencies and intensities. The symmetry group of an nn pair has been constructed by extending the group theory of matrix isolated molecule developed by Miller and $Decius.2$ The transition intensities due to four different mechanisms were calculated based on the earlier theoretical studies of Harris, Berlinsky, and $Hardy.3$ A discussion on the detailed analysis based on symmetry considerations, intensities calculations, and the new observations of the dependence of polarization of the infrared radiation on the intensities will be presented.