State-selected ion–molecule reactions with Coulomb-crystallized molecular ions in traps

State-selected Coulomb-crystallized molecular ions were employed for the first time in ion–molecule reaction studies using the prototypical charge-transfer process N 2 + + N 2 → N 2 + N 2 + as an example. By preparing the reactant ions in a well-defined rovibrational state and localizing them in space by sympathetic cooling to milliKelvin temperatures in an ion trap, state- and energy-controlled reaction experiments with sensitivities on the level of single ions were performed. The experimental results were interpreted with quasi-classical trajectory simulations on a six-dimensional potential-energy surface which provided detailed insight into translation-to-rotation energy transfer occurring during charge transfer between N2 and N 2 + .