THEORY OF THE Q BRANCH OF THE INFRARED SPECTRUM OF SOLID HYDROGEN

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Abstract
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.