Precision measurement of the hyperfine and fine structure for the lithium-6,7(+) 1s2s sulfur-3(-1) going to 1s2p phosphorus-3(-1,2) transition

Recent advances in atomic theory for two-electron ions have been made using variational techniques. Comparison of this theory with precision experiments is of importance for testing our fundamental understanding of two-electron atoms. In this experiment the hyperfine and fine structure splittings of the 1s2s 3S1 → 1s2p 3P1,2 transition of Li+ were investigated. A 5 keV Li+ beam was produced by a low-density electron-bombardment ion source. Approximately 1% of the 150 nA beam emerged from the source in the 1s2s 3S1 metastable state. A ring-dye laser beam that was frequency modulated by a 9.2 or 6.8 GHz electro-optic modulator was used to collinearly excite the ions. Photon-counting electronics recorded the fluorescence intensity as the laser frequency was scanned across the resonance. Each transition was excited by the various frequency sidebands of the modulated laser beam, which in turn permitted the scan to be calibrated. The linearity of the laser frequency scan was monitored by recording the transmission of part of the laser beam through a Fabry-Perot etalon. The measured 6Li+ hyperfine splittings are an order of magnitude more precise than existing data in the literature and the 7Li+ results resolve an 11 MHz (17sigma) discrepancy for the 3P1,2 fine structure interval as determined by two previous experiments. The results for 12 (all 13) measured hyperfine intervals are within 2sigma (3sigma) of the latest Hylleraas variational calculations.