The preparation of “cold” molecules and ions at temperatures T<<1 K has been one of the most exciting developments in chemical physics in recent years. A particularly interesting variant of cold molecules are cold trapped ions which localise in space and form ordered structures called “Coulomb crystals”. Under these conditions, it is possible to observe, address and manipulate single particles in a precisely controlled environment. This technology paves the way for the study and control of chemical reactions on the single-particle level, for the manipulation of the quantum state of single molecules, for spectroscopic measurements with an unprecedented accuracy and for new perspectives in quantum technology. Our work is devoted to exploring the unusual properties of cold molecules and ions and their various applications in both chemistry and physics.

Quantum control of cold molecular ions
We recently achieved the preparation of Coulomb-crystallized molecular ions in well-defined internal, i.e., rotational-vibrational, quantum states. These state-selected, cold and spatially localised molecules offer intriguing prospects for the quantum- and energy-control of chemical reactions as well as new systems for quantum technology. 
Read more: PRL 105, 143001 ; CPL 547, 1
Cold chemistry: ion-neutral hybrid traps
The new technologies used in our experiments allow us to manipulate both, the translational motion and the internal quantum state of molecules. These capabilities open up avenues for the study of chemical reactions at ultralow temperatures far lower than one Kelvin (“cold chemistry”). We are using combined (“hybrid”) traps for the simultaneous trapping of cold ions and cold atoms to study unusual chemical processes in this regime and unravel fine details of intermolecular interactions. 
Read more: PRL 107, 243202 ; PRL 109, 233202
Cold molecular ions on a chip
Surface-electrode ion traps represent a new type of trapping architecture in which all electrodes lie in a plane and the ions are trapped above the chip. This new generation of traps offers a greatly increased flexibility for manipulating, separating and shuttling of cold ions. We have recently extended this technology to molecular ions as a basis for our next-generation cold-molecular-ions experiments.
Read more: PRA 90, 023402
Controlled chemistry: conformer-specific chemical reactions 
The relationship between structure and reactivity is one of the central tenets of chemistry. In particular, complex molecules can exhibit a variety of conformational isomers which are difficult to isolate and study individually. Together with the group of J. Küpper (Uni Hamburg/CFEL), we have recently developed a method to study reactions of selected molecular conformations under single-collision conditions in the gas phase. Our approach relies on the spatial separation of distinct conformers in an inhomogeneous electric field and their selective reaction with a target of Coulomb crystallized ions.
Read more: Science 342, 98 ; JCP 142, 124202
Quantum technology and precision molecular spectroscopy
Building on the extremely high sensitivity achievable with Coulomb-crystal techniques, we have recently studied for the first time the “forbidden” infrared spectrum of a molecular ion. These spectra are of interest for precision measurements of molecular properties and the development of extremely precise “molecular clocks”. To this end, we are also developing techniques for precision spectroscopic measurements on single molecules using quantum-logic techniques.
Read more: PRA 85, 022308 ; Nature Phys.

image by Y.P. Chang (DESY)

A traveling wave Zeeman decelerator
A prominent, versatile method to produce cold molecules relies on the supersonic expansion of a seeded molecular gas, followed by a deceleration of the molecules of the so-formed beam. A new approach to the magnetic deceleration of supersonic beams is to generate a propagating wave of magnetic field. The fields provide real-time 3-dimensional confinement of the particles in low-field-seeking states. Our Zeeman decelerator avoids losses of molecules even at low forward velocities, prevents non-adiabatic transitions, and ideally matches a static magnetic trap.