THE VIBRATIONAL RAMAN SPECTRUM OF SOLID AND LIQUID HYDROGEN.

Author Institution: Department of Physics, University of TorontoThe frequencies of the vibrational Raman lines $Q_{1}(0)$ and $Q_{1}(1)$ of hydrogen have been measured for various ortho-para ratios in the solid at $13.5^{\circ}K$ and $2^{\circ}K$ and in the liquid. In all cases the Raman frequency is smaller $(7-12 cm^{-1}$) than for the free molecule. The frequency of the $Q_{1}(0)$ maximum changes linearly with the para concentration in the available range, 100-25\%. The change from the gas frequency can be explained as the super-position of two effects: isotropic molecular interactions which produce a shift independent of the para concentration, and vibrational coupling which produces a smaller shift varying linearly with the para concentration. The frequency of the $Q_{1}(1)$ line behaves similarly for ortho concentrations in the range 75-20\%. At lower concentrations the $Q_{1}(1)$ maximum breaks up into a number of closely spaced components, indicating that exciton excitation can no longer be sustained and that coupling occurs only with nearest ortho neighbours. The change in the frequencies of both the $Q_{1}(0)$ and $Q_{1}(1)$ lines with change in the temperature of the solid and with the transition from solid to liquid is very nearly independent of the ortho-para ratio. It is concluded from this that the vibrational coupling effect is almost the same in the liquid and solid