  We investigate the photoassociation dynamics of exactly two laser-cooled
$^{85}$Rb atoms in an optical tweezer and reveal fundamentally different
behavior to photoassociation in many-atom ensembles. We observe non-exponential
decay in our two-atom experiment that cannot be described by a single rate
coefficient and find its origin in our system's pair correlation. This is in
stark contrast to many-atom photoassociation dynamics, which are governed by
exponential decay with a single rate coefficient. We also investigate
photoassociation in a three-atom system, thereby probing the transition from
two-atom dynamics to many-atom dynamics. Our experiments reveal additional
reaction dynamics that are only accessible through the control of single atoms
and suggest photoassociation could measure pair correlations in few-atom
systems. It further showcases our complete control over the quantum state of
individual atoms and molecules, which provides information unobtainable from
many-atom experiments.
