  The role of electronic excitation in charge exchange chemical reactions
between ultracold Ca atoms and Ba$^+$ ions, confined in a hybrid trap, is
studied. This prototypical system is energetically precluded from reacting in
its ground state, allowing a particularly simple interpretation of the
influence of electronic excitation. It is found that while electronic
excitation of the ion can critically influence the chemical reaction rate,
electronic excitation of the neutral atom is less important. It is also
experimentally demonstrated that with the correct choice of the atom-ion pair,
it is possible to mitigate the unwanted effects of these chemical reactions in
ultracold atom-ion environments, marking an important step towards the next
generation of hybrid devices.
