Effect of orthohydrogen impurity on the absorption intensities of single transitions in a solid parahydrogen crystal

This paper provides insights for the accurate calculation of theoretical intensities of single transitions in a parahydrogen crystal containing trace amounts of orthohydrogen impurity. From the ratio of the experimental intensity to that of theoretical one (α˜exp/α˜th) we can deduce the value of the phonon renormalization factors, which are not easily amenable for calculations, a priori and are customarily set equal to unity. The ratio α˜exp/α˜th for S transitions varies over a wide range from 0.15 to 0.94. Different reasons are advanced for this wide variation and remedy suggested. A more careful experimental measurement of the intensity of single transitions in a parahydrogen crystal at different, carefully controlled, orthohydrogen concentrations is desirable. The accurate knowledge of the ratio α˜exp/α˜th will then lead to a detailed understanding of the theory of phonon renormalization factors in quantum solids.