Microwave absorption by ortho pairs in solid hydrogen

The microwave absorption spectra of ortho pairs in low ortho concentration samples of solid hydrogen is studied theoretically and experimentally. A theory is developed for several line broadening mechanisms and a comparison made with experiment. Phonon induced lifetime effects are calculated to be less than 2 MHz in the temperature range T=1.2-4.2 K but have not yet been observed experimentally. Inhomogeneous broadening due to isolated ortho molecules and to isotopic mass defect impurities is observed and found to be consistent with theory. Several new lines in the out-of-plane pair configuration are reported which confirm the assignments made previously, whereas two new lines in the next nearest neighbor configuration support a reassignment of these transitions. Also, three excited state transitions which fix the position of the 10 P level are reported and compared with the theoretical predictions of Harris et al. (1977) and Luryi and Van Kranendonk (1979).