  Assemblies of highly excited Rydberg atoms in an ultracold gas can be set
into motion by a combination of van-der-Waals and resonant dipole-dipole
interactions. Thereby, the collective electronic Rydberg state might change due
to non-adiabatic transitions, in particular if the configuration encounters a
conical interaction. For the experimentally most accessible scenario, in which
the Rydberg atoms are initially randomly excited in a three-dimensional bulk
gas under blockade conditions, we numerically show that non-adiabatic
transitions can be common when starting from the most energetic repulsive
BO-surface. We outline how this state can be selectively excited using a
microwave resonance, and demonstrate a regime where almost all collisional
ionization of Rydberg atoms can be traced back to a prior non-adiabatic
transition. Since Rydberg ionisation is relatively straightforward to detect,
the excitation and measurement scheme considered here renders non-adiabatic
effects in Rydberg motion easier to demonstrate experimentally than in
scenarios considered previously.
