The rotational temperature of polar molecular ions in Coulomb crystals

With MgH^+ ions as a test case, we investigate to what extent the rotational motion of smaller polar molecular ions sympathetically cooled into Coulomb crystals in linear Paul traps couples to the translational motions of the ion ensemble. By comparing results obtained from rotational state-selective resonance-enhanced photo-dissociation experiments with data from theoretical simulations, we conclude that the effective rotational temperature exceeds the translational temperature (< 100 mK) by more than two orders of magnitude, indicating a very weak coupling. In fact, the experimental results are consistent with the rotational temperature being in thermal equilibrium with the surrounding environment at ~300 K.