  Controlling the pathways and outcomes of reactions is a broadly pursued goal
in chemistry. In gas phase reactions, this is typically achieved by
manipulating the properties of the reactants, including their translational
energy, orientation, and internal quantum state. In contrast, here we influence
the pathway of a reaction via its intermediate complex, which is generally too
short-lived to be affected by external processes. In particular, the ultracold
preparation of potassium-rubidium (KRb) reactants leads to a long-lived
intermediate complex (K$_2$Rb$_2^*$), which allows us to steer the reaction
away from its nominal ground-state pathway onto a newly identified
excited-state pathway using a laser source at 1064 nm, a wavelength commonly
used to confine ultracold molecules. Furthermore, by monitoring the change in
the complex population after the sudden removal of the excitation light, we
directly measure the lifetime of the complex to be $360 \pm 30$ ns, in
agreement with our calculations based on the Rice-Ramsperger-Kassel-Marcus
(RRKM) statistical theory. Our results shed light on the origin of the two-body
loss widely observed in ultracold molecule experiments. Additionally, the long
complex lifetime, coupled with the observed photo-excitation pathway, opens up
the possibility to spectroscopically probe the structure of the complex with
high resolution, thus elucidating the reaction dynamics.
