The spectrum and term system of He-like O VII

The spectrum of He-like O VII has been studied at the ISX-A tokamak in Oak Ridge. In the region 15-140 angstrom about 20 O VII lines were measured. The results were combined with previous data to derive an energy level diagram for O VII. The ionization energy was determined to 5963078 +/- 175 cm-1. This value is in agreement with previous experimental results but the uncertainty has been significantly reduced. A comparison with theoretical calculations yields a value of 440 +/- 175 cm-1 for the QED contribution ("Lamb shift") to the 1s2 S-1(0) ground term energy, in good agreement with theoretical predictions.