  At ultralow energies, atoms and molecules undergo collisions and reactions
that are best described in terms of quantum mechanical wave functions. In
contrast, at higher energies these processes can be understood
quasiclassically. Here, we investigate the crossover from the quantum
mechanical to the quasiclassical regime both experimentally and theoretically
for photodissociation of ultracold diatomic strontium molecules. This basic
reaction is carried out with a full control of quantum states for the molecules
and their photofragments. The photofragment angular distributions are imaged,
and calculated using a quantum mechanical model as well as the WKB and a
semiclassical approximation that are explicitly compared across a range of
photofragment energies. The reaction process is shown to converge to its
high-energy (axial recoil) limit when the energy exceeds the height of any
reaction barriers. This phenomenon is quantitatively investigated for
two-channel photodissociation using intuitive parameters for the channel
amplitude and phase. While the axial recoil limit is generally found to be well
described by a commonly used quasiclassical model, we find that when the
photofragments are identical particles, their bosonic or fermionic quantum
statistics can cause this model to fail, requiring a quantum mechanical
treatment even at high energies.
