Observation of dipole-quadrupole interaction in an ultracold gas of Rydberg atoms

TL;DR

This study reports the direct excitation of cesium atom pairs to molecular states influenced by dipole-quadrupole interactions, revealing complex coupling mechanisms in ultracold Rydberg gases.

Contribution

First observation of dipole-quadrupole interactions in ultracold Rydberg atom pairs, supported by Stark spectroscopy and detailed potential modeling.

Findings

Identification of molecular resonances from dipole-quadrupole interactions

Support from Stark spectroscopy and potential modeling

Evidence of non-Born-Oppenheimer coupling in ultracold gases

Abstract

We observe the direct excitation of pairs of Cs atoms from the ground state to molecular states correlating asymptotically to $ns\,n'f$ asymptotes. The molecular resonances are interpreted as originating from the dipole-quadrupole interaction between the $ns\,n'f$ pair states and close-by $np\,np$ asymptotes ($22\leq n \leq 32$). This interpretation is supported by Stark spectroscopy of the pair states and a detailed modeling of the interaction potentials. The dipole-quadrupole interaction mixes electronic states of opposite parity and thus requires a coupling between electronic and nuclear motion to conserve the total parity of the system. This non-Born-Oppenheimer coupling is facilitated by the near-degeneracy of even and odd $L$ partial waves in the atom-atom scattering which have opposite parity.