Determination of quantum defect for the Rydberg P series of Ca II

Abstract

We present an experimental investigation of the Rydberg 23 P${}_{1/2}$ state of 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 Rydberg 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) ${\mathrm{cm}}^{-1}$, in agreement with the accepted value, and the quantum defect for the $n$ P${}_{1/2}$ states.

7 pages, 3 figures