Long-Range Molecular Resonances in a Cold Rydberg Gas

Abstract

We present evidence for molecular resonances in a cold dense gas of rubidium Rydberg atoms. Single UV photon excitation from the $5s$ ground state to $np$ Rydberg states ($n=50–90$) reveals resonances at energies corresponding to excited atom pairs $(n-1)d+ns$. We attribute these normally forbidden transitions to avoided crossings between the long-range molecular potentials of two Rydberg atoms. These strong van der Waals interactions result in avoided crossings at extremely long range, e.g., $\sim 58000$ times the Bohr radius ($a_0)$ for $n=70$.