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 manybody quantum mechanics 6,7 , whereas in chemistry, stateprepared molecular targets are of interest for uni-molecular reactions with coherent light fields 8,9 , for quantum-stateselected 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 lasercooling 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 + electronic groundstate 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 setups. At present there is a strong interest within the scientific community to produce and experiment with cold molecules. As standard laser-cooling schemes developed for atomic species cannot be applied to molecules, in the recent past tremendous effort has been put into developing schemes to cool molecules by other means. Several schemes for producing translationally cold and strongly bound neutral molecules in a single quantum state have been demonstrated. These include buffer-gas cooling of magnetic dipolar molecules in magnetic traps 15 , Stark deceleration and trapping of molecules with permanent 16,17 or induced 18 electric dipole moments, formation of molecules through Feshbach resonances followed by transfer to the vibrational ground state of electronic molecular potentials by stimulated Raman adiabatic passage schemes 19 and photoassociative molecule formation directly in the rovibrational ground state 20 or by transfer to the vibrational ground state through the application of shaped femtosecond laser pulses 21. With respect to molecular ions, the well-established technique of buffer-gas cooling is the only method that has so far led to the production of translationally as well as internally cold molecular ions 22. Although buffer-gas cooling is simple and generally applicable, it limits both the effective internal and translational temperature to the kelvin range, and owing to frequent collisions with the buffer-gas atoms it prevents strong spatial localization as well as utilization of internal coherences. Sympathetic cooling of trapped molecular ions, through the Coulomb interaction with laser-cooled atomic ions, has, on the other hand, proven highly effective for reaching translational temperatures in the millikelvin range, where the ions become spatially localized in the form of so-called Coulomb crystals 23. The sympathetic cooling scheme does, however, not significantly influence the molecular ion's internal degrees of freedom, owing to the very distant Coulomb interactions within the crystalline structures. ...

Isotope effects in reactions between Mg+ in the 3p{2}P{3/2} excited state and molecular hydrogen at thermal energies are studied through single reaction events. From only approximately 250 reactions with HD, the branching ratio between formation of MgD+ and MgH+ is found to be larger than 5. From an additional 65 reactions with H2 and D2 we find that the overall fragmentation probability of the intermediate MgH2+, MgHD+, or MgD2+ complexes is the same. Our study shows that few single ion reactions can provide quantitative information on ion-neutral reactions. Hence, the method is well suited for reaction studies involving rare species, e.g., rare isotopes or short-lived unstable elements.

Using the positively charged aniline ion ͑C 6 H 5 NH 2 + ͒ as a test molecule, we demonstrate that it is possible to study consecutive photodissociation of complex molecular ions at the single molecule level in an ion trap. When a single C 6 H 5 NH 2 + ion is exposed to laser light at 397 nm and 294 nm, direct or consecutive photodissociation leads to the production of a range of smaller polyatomic molecular ions such as C 5 H 6 + and C 3 H 3 + . The applied method is very versatile and can, e.g., be used in combination with free electron lasers or synchrotron radiation sources.