  We study cold heteronuclear atom ion collisions by immersing a trapped single
ion into an ultracold atomic cloud. Using ultracold atoms as reaction targets,
our measurement is sensitive to elastic collisions with extremely small energy
transfer. The observed energy-dependent elastic atom-ion scattering rate
deviates significantly from the prediction of Langevin but is in full agreement
with the quantum mechanical cross section. Additionally, we characterize
inelastic collisions leading to chemical reactions at the single particle level
and measure the energy-dependent reaction rate constants. The reaction products
are identified by in-trap mass spectrometry, revealing the branching ratio
between radiative and non-radiative charge exchange processes.
