Direct Laser Cooling of Molecules


Molecules at ultracold temperatures move at velocities close to a standstill and can be precisely controlled in their quantum state and their orientation in the lab frame. Therefore, their structure and collisional properties can be investigated with much higher precision, enabling fundamental tests of symmetries on the quantum level as well as ultracold chemistry. A dense gas comprised of many ultracold molecules would furthermore represent a system governed by quantum behaviour and long-range interactions allowing the experimentalists to simulate solid state behaviour with unprecedent control and precision, in order to investigate a multitude of phenomena such as superconductivity and new phases of matter.

Our research aims to cool down diatomic molecules from 10000K down to ultracold temperatures by the means of buffer gas cooling and direct laser cooling. Laser cooling however is not straightforward to implement for molecules due to their complex inner structure which includes vibration and rotation, so that cooling and slowing devices known from atomic physics cannot be transferred directly to molecules. Still much progress was achieved and molecular lasercooling has been shown.

The challenge for us is to refine molecular buffer gas and laser cooling further and to invent new cooling schemes which take the unique molecular structure into account and implement them in our lab. Recently, our group has demonstrated the possibility for Zeeman slowing of a molecular beam, which, combined with existing techniques for producing magneto-optical and magnetic traps for molecules, will enable us to reach the ultracold regime.


We developed a Type ll Zeeman slowing scheme which takes into account the unique complexity involved in lasercooling of molecules. As a proof of principle we first demonstrated our scheme on the analogous D1-Line of 39K. It turned out to work really well and nearly as good as the traditional atomic Zeeman Slower. This technique could boost current direct laser cooling experiments for molecules and we are currently starting to set up the actual molecular experiment. Where we want to combine our new Type ll Zeeman slowing scheme with a buffer gas beam source and a molecular magneto optical trap.  


We are currently setting up our buffer gas beam source which will be the starting point for our experiment. The molecules of interest will be produced via laser ablation of a molecular precursor target, after ablation the molecules have a high temperature on the order of 10000K. The ablation takes place in an cryogenic environment at a temperature of approximately 4 K. Cold helium is flushed over the molecular target in a so called buffer gas cell. Through collisions the hot molecules scatter with the cold helium and will be subsequently cooled down to 4 K and a molecular beam is formed. This buffer gas cooling ensures that the molecules of interest are in a defined rotational and vibrational state and the molecular beam has a moderate velocity of approximately 140 m/s. These precooled molecules will then be the starting point for molecular laser cooling.