Observation of second- and higher-order electric quadrupole interactions with an atomic ion

Abstract

We observe an atomic ion's response to an electric quadrupole field to second and higher orders; this arises from the ion's electric quadrupole polarizability and hyperpolarizabilities. We probe a single ^{88}Sr^{+} ion which is confined in the electric fields of a Paul trap and excited to Rydberg states. The quadrupolar trapping fields cause atomic energy level shifts and give rise to spectral sidebands. The effects on Rydberg S_{1/2} states are described well by second-order perturbation calculations. The stronger effects on Rydberg P_{1/2} states are captured by full diagonalization of the Hamiltonian.