Observation of highly vibrationally excited trilobite molecules stabilized by non-adiabatic coupling

We report on the observation of highly excited ($\nu \sim 100)$ vibrational states of a trilobite ultralong-range Rydberg molecule. These states manifest spectroscopically as a regularly spaced series of peaks red-detuned from the $25f_{7/2}$ dissociation threshold. A theoretical assignment of these levels based on a purely adiabatic approach is incompatible with the measured level spacing. Even their existence is in question in the adiabatic picture, as they are destabilized by the strong perturbation caused by the $p$-wave scattering shape resonance. Their observed stability therefore hinges on the almost complete suppression of this adiabatic decay pathway. This is predicted to occur for a select few Rydberg states where the avoided crossing between trilobite and butterfly potential curves occurs in the close vicinity of a node of the hydrogenic radial wave function with zero angular momentum. We confirm this by searching for excited states near the $27f_{7/2}$ threshold; in accordance with this prediction, none are observed. This is the first direct measurement of beyond-Born-Oppehnheimer physics in long-range Rydberg molecules, and paves the way for future experiments to access and manipulate high-lying states for time-dependent phenomena.