On the Quenching of Molecular Rotation of Ortho-Hydrogen in Solid State

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By using the data measured by Hill and Ricketson, we can express the anomalous specific heat C_V of solid hydrogen at sufficiently high temperatures approximately as C_VT^2 / R=αc+βc^2, α=1.1, β=15.7, where c is the concentration of ortho-hydrogen molecules, T the absolute temperature, and R the gas constant. It is an aim of this paper to inquire into the origin of these two terms. Through calculations by the method of moment expansion, the magnitude of β can be explained from the quadrupolar interactions among ortho-molecules, while the anisotropic nature of the valence and van der Waals forces is shown to be effective only as corrective terms. On the other hand, the appearance of α shows that the molecular rotation of an ortho-molecule surrounded by para-molecules should also be quenched. A possible mechanism of this phenomenon may be the rotation-vibration coupling, for which the valence and van der Waals forces should be exclusively responsible. However, we are not yet able to get a complete understanding of this quenching process. Deviation from the above formula, i.e.the linear dependence on c of the the quantity C_VT^2 / R at temperatures not high enough is also derived and compared with experiment. Further, a more exact treatment than the moment expansion method is given for the case of low ortho-concentrations and is applied to the case of 7% ortho.
理論物理学刊行会の論文