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

By using the data measured by Hill and Ricketson, we can express the anomalous specific heat CV of solid hydrogen at sufficiently high temperatures approximately as CVT2 / R = αc + βc2, 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 CVT2/R at temperatures not high enough is also derived and compared with experiment. Further, a more exact treatement than the moment expansion method is given for the case of low ortho-concentrations and is applied to the case of 7% ortho.

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