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Laser cooling of molecules

Laser cooling has been one of the versatile tools in atomic physics for more than 50 years. Lasers are used to slow down atoms by a kind of photon wind and these can then be held in so-called magneto-optical traps (MOTs). This enables high-precision spectroscopic investigations, for example in order to investigate the physics beyond the standard model.

For cooling to be successful, the atoms must be able to absorb photons from a certain energy level again and again and spontaneously return almost exclusively to the same level under photon emission.

Such closed optical cycles were initially considered impossible in molecules because they have more degrees of freedom, especially vibrational degrees of freedom, and can therefore fall down to various levels after absorption. However, in 2004 Michael Di Rosa established criteria for the laser coolability of molecules and diatomic candidates such as CaH were proposed for this.

In 2010 we have developed a special classification of unpaired electrons in order to find easily suitable diatomic molecules for laser cooling. In particular we have identified RaF as a promising candidate, that will also be highly interesting for the study of fundamental symmetries. Recently - together with colleagues from CERN - we were able to obtain laser spectroscopic information about this highly exciting molecule.

In 2010, David DeMille's group successfully lasercooled the molecule SrF for the first time. Since then, other diatomic systems have also been cooled by lasers, such as YO, CaF und YbF.

Due to our general classification of unpaired electrons we were able to quickly identify even polyatomic molecules for laser cooling. We have pointed to this principal possibility already in 2011 at a Conference in Boston in the United States of America. At a conference in Monte Verita in Switzerland in 2014, we have proposed explicit candidates such as CaOH for laser cooling and also presented the general concept for identification of suitable polyatomic molecules for the first time. This proposal was enthusiastically received. Other groups therefore have taken up the idea and turned towards laser cooling of polyatomic molecules. In April 2015 we have stored our ideas and the simple theoretical concepts on the arXiv preprint server and in February 2016 the related work was finally published in Physical Review Letter. And today it is considered generally accepted and almost taken for granted that even polyatomic molecules can be cooled with lasers.