Resolving closely spaced hyperfine levels in the 3P3/2 state of 23Na

We have resolved closely spaced hyperfine levels in the 3P(3/2) state of Na-23 with a technique of coherent-control spectroscopy that uses co-propagating beams. The probe beam is locked to one hyperfine transition, while the control beam is scanned across a neighbouring transition. An acousto-optic modulator placed in the path of the control beam provides calibrated frequency offsets for measuring the hyperfine intervals. We thus obtain precise values for the hyperfine constants in the 3P(3/2) state: A = 18.530(3) MHz and B = 2.721(8) MHz, which improve previous values significantly