Physics of Rydberg atoms in inhomogeneous electric fields

Charge-neutral interactions are of fundamental interest in physics and chemistry, and recent experiments are beginning to explore them within the quantum regime. Rydberg excitations offer a route toward tunable atom-ion interaction strengths and the sensitivity of Rydberg atoms to electric fields enables them to form long-range weakly-bound diatomic molecular ions. These dimers bind on micrometer length scales due to the Rydberg's large induced dipole moment, which is the leading-order term in the ion-neutral interaction series. Our work explores the role of the higher-order multipole terms in this interaction series, focusing on emergent physics as a result of the ion's inhomogeneous electric field. Surprisingly, we find that the Rydberg atom's quadrupole may compensate the enormous Coulomb repulsion between a pair of cations, forming a metastable doubly-charged trimer. Additionally, the quadrupole interaction term introduces an unexpected dependence on the sign of the ion's charge, which can significantly alter non-adiabatic couplings between potential energy curves. These modified couplings would not only affect vibrational dynamics, but also lifetimes of bound dimers.