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The theory of the zero-phonon Q branch of the infrared spectrum of solid hydrogen is developed. At low ortho-concentrations the spectrum can be analyzed in terms of transitions due to single and pairs of neighboring ortho-molecules. The Q1(1) transitions due to single ortho-molecules give rise to a sharp absorption line, the Q2(0) transitions to a band with a total width of about 3 cm−1 and a line profile following approximately a Δν3/2 law. The integrated intensities of these lines are calculated and compared with the experimental data. The Q1(1) and Q1(0) transitions due to pairs of ortho-molecules give rise to a number of components resulting from the removal of the ninefold degeneracy of the orientational state of the pair of ortho-molecules by the quadrupole–quadrupole interaction. The intensities and frequency separations of these components are calculated. Finally, a calculation is presented of the total integrated intensity of the zero-phonon Q branch at arbitrary ortho-concentrations. These theoretical results are of importance for the study of the percolation problem in solid hydrogen, and of the order-disorder transition taking place at high ortho-concentrations.

The theory of the rotational and vibrational energy bands in solid hydrogen, developed previously, is applied to the interpretation of the infrared and Raman spectra of solid parahydrogen. A comparison of the theory with the experimental results yields information about the nature of the rotational and vibrational motions of the molecules in the solid, and about the anisotropic intermolecular forces. A calculation of the intensity of the infrared rotational and vibrational lines is given, which is based on the induction mechanisms introduced previously to explain the induced absorption in gaseous hydrogen. Satisfactory agreement with the experimental values is obtained. The importance of the interaction between other than nearest neighboring molecules, and of the interference effects affecting the single transitions, is pointed out. A frequency analysis is given of the infrared and Raman rotational and vibrational lines in the solid, and a consistent theory of the shifts and splittings of the various lines is obtained. Empirical values of the rotational and vibrational coupling constants are derived and compared with the theoretical values.