  Light-assisted reactive collisions between laser-cooled Ba+ ions and Rb atoms
were studied in an ion-atom hybrid trap. The reaction rate was found to
strongly depend on the electronic state of the reaction partners with the
largest rate constant (7(2) x 10^-11 cm^3 s^-1) obtained for the excited
Ba+(6s)+Rb(5p) reaction channel. Similar to the previously studied Ca+ + Rb
system, charge transfer and radiative association were found to be the dominant
reactive processes. The generation of molecular ions by radiative association
could directly be observed by their sympathetic cooling into a Coulomb crystal.
Potential energy curves up to the Ba+(6s)+Rb(5p) asymptote and
reactive-scattering cross sections for the radiative processes were calculated.
The theoretical rate constant obtained for the lowest reaction channel
Ba+(6s)+Rb(5s) is compatible with the experimental estimates obtained thus far.
