Fine structure of the metastable a3Σu+ state of the helium molecule

We have measured the spin—spin, electronic fine structure of the $a^3{\Sigma }_u^{+}$ metastable state of He₂ by means of a molecular beam, magnetic resonance apparatus, with the flowing afterglow of a pulsed, helium discharge as the source. The zero field separations in the N=1 and N=3 rotational levels, probably in the ν=0 vibrational state, are (in MHz) N=1; J=0 ↔ J=1:2199.968 ± 0.01, J=1 ↔ J=2: 873.668 ± 0.0007; N=3: J=2 ↔ J=3: 1323.911±0.006, J=3↔J=4: 964.992±0.006, where the errors quoted are three standard deviations of the mean. From these measurements and from a separate determination of the level order, the interaction constants for these states are the following (in MHz): spin—spin: λ:−1098.773 ±0.005(N=1), −1096.803±0.004(N=3), spin‐rotation: γ: −2.421±0.003(N=1), −2.414±0.001(N=3), where the errors are 3σ.