Rotational laser cooling of vibrationally and translationally cold molecular ions

Molecular targets prepared in well-defined quantum states play an essential part in a wide range of fields, from metrology to astrochemistry. Now, MgH + ions have been prepared in their lowest vibrational and rotational level using a laser-cooling scheme. This provides a fresh approach for exploring such phenomena and applications experimentally. Stationary molecules in well-defined internal states are of broad interest for physics and chemistry. In physics, this includes metrology 1 , 2 , 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 , 11 , 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 Σ + 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.