Weak features in the infrared spectum of solid para-hydrogen containing a small amount of orth-hydrogen impurity were measured with high resolution (0.01–0.036 cm −1 ) and high sensitivity using a Fourier transform spectrometer. Effective absorption path lengths in the solid of 3.75 cm (for the range 4 000–10 000 cm −1 ) and 15.0 cm (for the range 5000–8000 cm −1 ) were used to observe the discrete zero-phonon rovibrational transitions U 1←0 (0), U 1←0 (1), and W 1←0 (0), for which accurate line positions and intensity parameters are given. Line positions for the associated satellite spectra for each of those transitions and for the Q 1←0 (0) and Q 1←0 (1) transitions were also determined. New features in the satellite spectra of the Q 1←0 (0), Q 1←0 (1), U 1←0 (0), and U 1←0 (1) transitions are reported. A novel ortho-hydrogen to para-hydrogen converter allowed convenient variation of the temperature for conversion. A multipass optical system external to the head of the cryostat was designed to extend the absorption path length through the hydrogen crystal.
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Steinhoff et al. (1994) studied this question.