2004ApJ...610..813B

The most accurate way to measure the energy levels for the O II 2p³ ground configuration has been from the forbidden lines in planetary nebulae. We present an analysis of modern planetary nebula data that nicely constrain the splitting within the ²D term and the separation of this term from the ground ⁴S_3/2level. We extend this method to H II regions using high-resolution spectroscopy of the Orion Nebula, covering all six visible transitions within the ground configuration. These data confirm the splitting of the ²D term while additionally constraining the splitting of the ²P term. The energies of the ²P and ²D terms relative to the ground (⁴S) term are constrained by requiring that all six lines give the same radial velocity, consistent with independent limits placed on the motion of the O⁺ gas and the planetary nebula data.