Observation of spin-structure of ultralong-range Rydberg molecules.

We present an experimental and theoretical study of spin coupling effects in a complex Rydberg molecular energy level structure. Specifically, we explore $^{87}$Rb$_2$ bound states below the $16P_{3/2}+5S_{1/2}$ dissociation threshold. The given complexity is due to the $p$-wave shape resonance which cuts through the potential wells leading to avoided level crossings. As our analysis will show, the spectra feature both characteristics of diabaticity and adiabaticity. For the measurements, a detection method for Rydberg molecules is used, which relies on elastic collisions of ionic products with neutral atoms in a combined trap. We observe characteristic multiplet line structures within individual vibrational states. By carrying out model calculations these multiplet structures can be explained as arising from a combination of spin-spin and spin-orbit coupling mechanisms. In our calculations, also spin-orbit interaction connected to the relative orbital angular momentum between the Rydberg electron and the ground state perturber is taken into account. We record the progression of different multiplet structures within their respective vibrational manifold. For an unambiguous assignment of resonance lines, we compare data sets obtained for different initial atomic hyperfine states. Our investigation of a variety of so far unresolved level structures paves the way for in-depth studies of subtle interaction mechanisms in Rydberg molecules, which manifest themselves in highly-complex potential energy curves.