Observation of new lines in the microwave spectrum of ortho-H2 pairs in solid hydrogen

Abstract

New microwave absorption spectra of ortho pairs in low ortho concentration samples of solid hydrogen are reported. Three excited state transitions which fix the position of the 10 Γ level for both types of nearest neighbour pairs are reported, and compared with the theoretical predictions of Harris, Berlinsky, and Hardy and Luryi and Van Kranendonk. Several new lines from the out-of-plane configuration are reported which confirm previous assignments. In contrast, two new lines from the next nearest neighbour spectrum support a reassignment of transitions for this configuration. A fit to this data determines several of the anisotropic potential parameters for next nearest neighbours. The improvements in the microwave spectrometer essential to obtaining the new results are also described. Finally, we compare our results to the most recent theoretical potential parameters of Schaefer and Meyer. A long standing discrepancy between theoretical and experimental values for the phonon renormalizationof Γ appears to be resolved by use of the new potential which has a substantial (~10%) non-quadrupolar contribution to ε4(R).