Stark effect in argon Rydberg states

We have applied the collinear laser spectroscopy technique to investigate the Stark structure of argon Rydberg states in the vicinity of the n=18 hydrogenic manifold below the first ionization limit (3${\mathit{p}}^5$ ${}_{}{}^2{}_{}{}^{}$ ${\mathit{P}}_{3/2}$ ionic core). Stark spectra have been recorded for electric fields from 0 up to 120 V/cm. Although the studied energy range is only 4 ${\mathrm{cm}}^{\mathrm{-}1}$ wide, it is sufficient to observe how the 18f level progressively merges into the hydrogenic linear Stark structure arising from the n=18 (l≥4) manifold. The high resolution of the method allowed us to observe several fine-structure components for each Stark level. The complexity of these spectra mainly originates in the electrostatic interaction of the Rydberg electron with an anisotropic ionic core. Using jK coupling, a calculation is developed that quantitatively fits the observed spectra. The energy positions of the lines, as well as their relative intensities, are accurately reproduced by this calculation. The best fit between theory and experiment is obtained when we take into account the presence of the odd-parity perturber state 18d[3/2${]}_1$, which is known to lie within the studied energy range.