Sympathetic Cooling of Complex Molecular Ions



One of the current themes of quantum physics is the creation of ultracold (< 1 K) molecules to study molecule-molecule, molecule-atom, and light-molecule interactions, where new interactions and vastly increased spectroscopic precision are expected. Due to the lack of closed optical transitions most molecular ion species cannot be laser-cooled directly. A powerful method overcoming this obstacle is sympathetic cooling. Here, an ensemble of directly laser-cooled atomic ions can be used to translationally cool an ensebmle of ions of any species via their mutual Coulomb interaction. Thereby sympathetic cooling allows to cool molecular ions with masses up to several 10,000 amu, for example organic molecules like dyes, amino acids or proteins.

Experimental Setup:

Figure 1: Experimental setup (the laser sources are not shown).


Our experimental setup (see figure 1) for preparing ulctracold molecular ions consists of an Electro Spray Ionization (ESI) ion source to transfer molecules from solution into the gas phase. Its output ion beam contains the molecular ions at different charge states and also complexes with some solvent molecules. In order to select the desired species a quadrupole mass filter is used. With a 2 m long rf ion guide the selected molecular ions are transferred through a differential vacuum system into a UHV chamber, where a linear Paul trap is placed. Trapped and stored together with laser-cooled barium ions in the Paul trap, the molecular ions can be sympathetically cooled down to temperatures below 100 mK. For spectroscopic analysis this state can be maintained, in principle for many hours.

If you are interested in more details on the experimental setup you can check our poster about Sympathetic cooling of complex molecular ions to milli-Kelvin temperatures.

Experimental Results:

At sufficiently low temperatures the trapped ion plasma undergoes a phase transition to an ordered state, a so-called Coulomb crystal. Using an intensified CCD camera we can image the barium ions:

Figure 2: Different structures of pure barium Coulomb crystals in our Paul trap.

However, an image of a multi-species atomic-molecular Coulomb crystal will only show the fluorescing laser-cooled barium ions. But as the environment of the trap apparatus is well-defined, we can deduce further information about the other species with the help of molecular dynamics simulations.

Figure 3: Simulation of a three-species Coulomb crystal containing 830 laser-cooled 138Ba+ ions (blue) at a temperature of 25 mK, 420 sympathetically cooled ions of barium isotopes (red) and 200 protonated AlexaFluor350 molecules (AF350, green) at 120 mK.

If you are interested in more details on our molecular dynamics simulations click here.