We present an experimental investigation of the Rydberg 23 P 1/2 state of single, laser-cooled 40 Ca + ions in a radiofrequency ion trap. Using micromotion sideband spectroscopy on a narrow quadrupole transition, the oscillating electric field at the ion position was precisely characterised, and the modulation of the Ryd-berg transition due to this field was minimised. From a correlated fit to this P line and previously measured P and F level energies of Ca II, we have determined the ionization energy of 95 751.916(32) cm −1 , in agreement with the accepted value, and the quantum defect for the n P 1/2 states.
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