  Atoms and molecules often react at different rates depending on their
internal quantum states. Thus, controlling which internal states are populated
can be used to manipulate the reactivity and can lead to a more detailed
understanding of reaction mechanisms. We demonstrate this control of reactions
by studying the excited state reaction reaction Ca$^+$ + O$_2$ $\rightarrow$
CaO$^+$ + O. This reaction is exothermic only if Ca$^+$ is in one of its
excited electronic states. Using laser-cooling and electrodynamic trapping, we
cool and trap Ca$^+$ at millikevin temperatures for several minutes. We can
then change the fraction of time they spend in each of the two excited states
by adjusting the detunings of the cooling lasers. This allows us to disentangle
the reactions that begin with Ca$^+$ in the $^2$P$_{1/2}$-state from the ones
where Ca$^+$ is in the $^2$D$_{3/2}$-state. Using time-of-flight mass
spectrometry, we determine independent reaction rate constants for Ca$^+$ in
both electronically excited quantum states.
