Energy shifts and forbidden transitions in H+2 due to electronic g/u symmetry breaking

A full account is given of calculations and measurements of transition frequencies and intensities of the forbidden pure rotation transition (v = 19, N = 1)-(v = 19, N = 0) in the ground electronic state (1ssigma(g)) of H-2(+). The transition has measurable intensity because of ortho-para mixing that arises from electronic g/u symmetry breaking caused by the Fermi contact hyperfine interaction. Measurements of the transition were made in both single and double resonance using a fast ion beam/microwave spectrometer. The transition frequency was determined to be at 14 961.7 +/- 1.1 MHz (95% confidence, 5 measurements), in excellent agreement with the theoretical prediction of 14 960 +/- 3 MHz. The intensity of the transition relative to the allowed 1ssigma(g)(v = 19, N = 1)-2psigma(u) (v = 0, N = 2) transition was estimated from the available measurements to be 8000, in reasonable agreement with the theoretically predicted value of approximate to 3000.