Rotational laser cooling of vibrationally and translationally cold molecular ions

Stationary molecules in well-defined internal states are of broad interest for physics and chemistry. In physics, this includes metrology(1-3), quantum computing(4,5) and many-body quantum mechanics(6,7), whereas in chemistry, state-prepared molecular targets are of interest for uni-molecular reactions with coherent light fields(8,9), for quantum-state-selected bi-molecular reactions(10-12) and for astrochemistry(12). Here, we demonstrate rotational ground-state cooling of vibrationally and translationally cold MgH(+) ions, using a laser-cooling scheme based on excitation of a single rovibrational transition(13,14). A nearly 15-fold increase in the rotational ground-state population of the X(1)Sigma(+) electronic ground-state potential has been obtained. The resulting ground-state population of 36.7 +/- 1.2% is equivalent to that of a thermal distribution at about 20 K. The obtained cooling results imply that cold molecular-ion experiments can now be carried out at cryogenic temperatures in room-temperature set-ups.