FREQUENCY-DEPENDENCE OF RAMAN-ACTIVE PHONONS IN SOLID HCP HYDROGEN ON ORTHO-PARA CONCENTRATION AND TEMPERATURE

A Raman scattering study of the phonon frequencies in disordered hcp solid hydrogen as a function of ortho-para concentration and temperature is presented. A crystal has been studied under isochroic conditions at a pressure of the order of 1000 bar to remove volume expansion effects present in previous zero-pressure studies. Results show that the phonon frequencies still depend weakly on the ortho concentration. This is interpreted as a small, but substantially larger dependence of the isotropic pair potential on the rotational state than has been believed to be the case. Anharmonic thermal frequency shifts are found to be in good agreement with theory. The ortho-para conversion rate of H2 at a molar volume of 18.45 cm3/ mole was determined. An unexplained change in the quadratic rate constant as a function of concentration has been observed. © 1979 Plenum Publishing Corporation.