  The level structure of negative-ions near the electron detachment limit
dictates the low-energy scattering of an electron with the parent neutral atom.
We demonstrate that a single ultracold atom bound inside a Rydberg orbit
forming an ultralong-range Rydberg molecule provides an atomic-scale system
which is highly sensitive to electron-neutral scattering and thus allows for
detailed insights into the underlying near-threshold anion states. Our
measurements reveal the so far unobserved fine structure of the $^3P_J$ triplet
of Rb$^-$ and allow us to extract parameters of the associated $p$-wave
scattering resonances which deviate from previous theoretical estimates.
Moreover, we observe a novel alignment mechanism for Rydberg molecules mediated
by spin-orbit coupling in the negative ion.
