Metastable doubly-charged Rydberg molecules

TL;DR

This paper predicts the existence of metastable, long-range Rydberg trimers involving a rubidium atom and two cations, stabilized by quadrupole interactions and exhibiting specific stability and lifetime characteristics depending on the principal quantum number.

Contribution

It introduces a new class of metastable Rydberg molecules stabilized by quadrupole interactions, distinct from previously observed dipole-flipping ion-Rydberg dimers.

Findings

Existence of metastable vibrationally-bound $^{87}$Rb$_3^{2+}$ states predicted.

Stability highly dependent on the principal quantum number $n$.

States are expected to exist for $n$ between 24 and 35, with lifetimes influenced by tunnelling and radiative decay.

Abstract

H$_3^{2+}$ is a one-electron system with three positive nuclei and is known to be unstable in its electronic ground-state. We examine an analogous one-electron system composed of a $^{87}$Rb Rydberg atom interacting with a pair of cations and predict the existence of metastable vibrationally-bound states of $^{87}$Rb$_3^{2+}$. These molecules are long-range trimers whose stability rests on the presence of core-shell electrons and favourable scaling of the Rydberg atom's quadrupole moment with the principal quantum number $n$. Unlike recently observed ion-Rydberg dimers, whose binding is due to internal flipping of the Rydberg atom's dipole moment, the binding of $^{87}$Rb$_3^{2+}$ arises from the interaction of the ions with the Rydberg atom's quadrupole moment. The stability of these trimers is highly sensitive to $n$. We do not expect these states to exist below $n=24$ and for $n \leq 35$, their lifetime is limited by tunnelling of the Rydberg electron. In contrast, at very large $n$ the lifetime will be limited by tunnelling of the vibrational wavepacket. In between these limits, we expect a range of bound states at intermediate $n$ for which both tunnelling rates are smaller than the radiative decay rate of the Rydberg state.