  Femtochemistry techniques have been instrumental in accessing the short time
scales necessary to probe transient intermediates in chemical reactions. Here
we take the contrasting approach of prolonging the lifetime of an intermediate
by preparing reactant molecules in their lowest ro-vibronic quantum state at
ultralow temperatures, thereby drastically reducing the number of exit channels
accessible upon their mutual collision. Using ionization spectroscopy and
velocity-map imaging of a trapped gas of potassium-rubidium molecules at a
temperature of 500~nK, we directly observe reactants, intermediates, and
products of the reaction $^{40}$K$^{87}$Rb + $^{40}$K$^{87}$Rb $\rightarrow$
K$_2$Rb$^*_2$ $\rightarrow$ K$_2$ + Rb$_2$. Beyond observation of a long-lived
energy-rich intermediate complex, this technique opens the door to further
studies of quantum-state resolved reaction dynamics in the ultracold regime.
