  We employ collisions of individual atomic cesium (Cs) impurities with an
ultracold rubidium (Rb) gas to probe atomic interaction with hyperfine- and
Zeeman-state sensitivity. Controlling the Rb bath's internal state yields
access to novel phenomena observed in inter-atomic spin-exchange. These can be
tailored at ultra-low energies, owing to the excellent experimental control
over all relevant energy scales. First, detecting spin-exchange dynamics in the
Cs hyperfine state manifold, we resolve a series of previously unreported
Feshbach resonances at magnetic fields below 300 mG, separated by energies as
low as $h\times 15$ kHz. The series originates from a coupling to molecular
states with binding energies below $h\times 1$ kHz and wave function extensions
in the micrometer range. Second, at magnetic fields below $\approx 100\,$mG, we
observe the emergence of a new reaction path for alkali atoms, where in a
single, direct collision between two atoms two quanta of angular momentum can
be transferred. This path originates from the hyperfine-analogue of dipolar
spin-spin relaxation. Our work yields control of subtle ultra-low-energy
features of atomic collision dynamics, opening new routes for advanced
state-to-state chemistry, for controlling spin-exchange in quantum many-body
systems for solid state simulations, or for determination of high-precision
molecular potentials.
