Observation of s-p Coupling in Ultralong-Range Rydberg Molecules

arXiv:2608.09950v2 Announce Type: replace-cross Abstract: Ultralong-range Rydberg molecules, formed by the interaction between a highly excited Rydberg atom and a ground-state atom, provide a unique platform for exploring quantum phenomena spanning nanometer- to micrometer distances as well as exotic few-body interactions. The molecular binding arises from electron-atom scattering, which is conventionally described by an independent s- and p-wave channels picture. Here we report the experimental observation of s-p coupling in Rb(nS)-Rb(5S) ultralong-range Rydberg molecules via high-resolution photoassociation spectroscopy. By varying the principal quantum number n from 31 to 36, we resolve an avoided crossing between the s- and p-wave molecular states: whenever a vibrational level bound by the p-wave well approaches one bound by the s-wave well in energy, the two states are mixed by the off-diagonal coupling U_sp, and the shifted s-wave level is identified, which is consistent with theoretical predictions. This mixing is most pronounced for low-node p-wave molecules, whose binding energies lie closer to those of the s-wave molecules. The resulting level repulsion and redistribution of spectral intensity reveal the mixed s-p character of ultralong-range Rydberg molecules and provide a more precise molecular spectroscopy.