Observation of $\Pi$-symmetry ultralong-range Rydberg molecules

Abstract

We observe weakly-bound $\Pi$-symmetry electronic states in the spectroscopy of $^{87}$Rb$(nP_{3/2})$+$^{87}$Rb($5S_{1/2}$) ultralong-range Rydberg molecules. We detect these molecules in Rydberg states having principal quantum number $13\le n \le 16$. Their $\Pi$-state character is unambiguously identified via their observed multiplet structure: the $2F+1$ magnetic sublevels of the ground-state rubidium atom separate, as in the Zeeman effect, because of the spin-spin coupling between the Rydberg and valence electrons. We find a rapid decrease in the molecular binding energy $\propto (n-\mu_{P_{3/2}})^{-11}$, where $\mu_{P_{3/2}}$ is the quantum defect, indicating that the low-$n$ regime of Rydberg states is ideally suited for studies of $\Pi$-symmetry molecules. Our observations are in good agreement with Green's function-based calculations for $14\le n\le 16$, with poorer agreement for $n=13$ hinting at the beginning of a breakdown of the Fermi pseudopotential approach at low $n$.