THE INFRARED FUNDAMENTAL BAND OF LIQUID AND SOLID HYDROGEN

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Abstract
The infrared fundamental band of liquid and solid hydrogen was investigated over a range of para-concentrations from 25% to 100% with a prism spectrometer and, in part, with a grating spectrometer at a resolution of ~0.2 cm−1. The spectrum of the solid shows (a) comparatively sharp Q, S(0), and S(1) lines due to quadrupolar interaction, (b) broad bands interpreted as combination tones of the molecular frequencies with the lattice frequencies (phonon spectra), and (c) weak double transitions of the type S1(0) + S0(0). At high resolution the quadrupolar S(0) and S(1) groups show weak single transitions, S1(0) and S1(1), and much stronger double transitions of the type Q1(J) + S0(J), J = 0,1. In solid parahydrogen the lines become very sharp and the double transition, Q1(0) + S0(0), shows a complex structure; the observations are in good agreement with the theory of the rotational and vibrational levels of solid parahydrogen by Van Kranendonk. The quadrupolar Q branch shows a structure which is interpreted as double transitions of the type Q ± δi where the δi are the small changes in energy due to the orientational transitions of two ortho-molecules. The phonon spectra show a maximum at the Debye temperature of the solid, and, at higher resolution, a structure indicative of the various branches of the lattice frequencies. A long extension of the phonon spectrum towards high frequencies is probably due to multiple phonon creation. Double transitions, for which the cancellation principle in induced absorption does not apply, account for at least 98% of the intensity of the spectrum of the solid.
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