Rydberg states of rubidium in crossed electric and magnetic fields

The results of an experimental study of the Rydberg spectrum of rubidium in crossed electric and magnetic fields are reported and compared with theoretical predictions. The present investigations have been conducted in the low-field regime in which the paramagnetic and linear Stark interactions are perturbations to the Coulomb energy. When quantum-defect corrections are negligible, the SO(4) symmetry of the Coulomb interaction organizes the hydrogenic energy spectrum according to the law ${\mathit{E}}_{\mathit{n}}$,k=-R/${\mathit{n}}^2$+ħk(${\mathrm{\omega }}_{\mathit{L}}^2$+${\mathrm{\omega }}_{\mathit{S}}^2$ ${)}^{1/2}$, where k is an integer such that -(n-1)≤k≤(n-1), and ${\mathrm{\omega }}_{\mathit{L}}$,${\mathrm{\omega }}_{\mathit{S}}$, are, respectively, the Larmor and linear Stark frequencies. The experimental evidence for such a quantization is presented in rubidium Rydberg series using Doppler-free two-photon spectroscopy. The role of quantum defects is incorporated into the theory and experimentally demonstrated.