UNIC · Ultracold negative ions by laser cooling

Laser cooling is a well-established technique for the creation of ultracold particle ensembles in beams or traps. Over the past 30 years, it has become an indispensable tool in atomic physics and has opened many exciting new research fields. Both in positive atomic ions and in neutral atoms, the valence electron is bound in a Coulomb potential. The resulting infinite series of excited states provides a wide choice of suitable cooling transitions in many ionic and atomic systems. Surprisingly, laser cooling of negative atomic ions has never been achieved. The binding of the valence electron in these systems is based on electron electron correlation effects, which drop off quickly as the excess electron is removed from the neutral core. Consequently, anions are easily neutralized and only a few of them have excited levels. When excited states do occur, they are usually sub-levels of the ground state, meaning that transitions between the ground and excited state are weak and laser cooling would take prohibitively long. However, only a few years ago, a strong transition between the ground state and an opposite-parity excited state was found in the negative osmium ion. With this discovery, the laser cooling of atomic anions has finally come into reach. High-resolution optical spectroscopy on negative osmium has been carried out by the applicant, confirming the existence of a potential laser cooling transition. The aim of the proposed project is the first-ever demonstration of atomic-anion laser cooling. Ultimately, laser-cooled atomic anions could be used to cool any other negative-ion species by confining them simultaneously in a trap. The proposed technique is therefore applicable to a wide range of research fields in which ultracold negative ions are required.