The Fribourg Atomic Physics group (FRAP) has a world-leading expertise in the study of atomic and molecular defects in solid helium matrices. This unique variant of matrix isolation spectroscopy in quantum solids is closely related to defect spectroscopy in heavier rare gas and hydrogen solids, in doped helium nano-droplets and in superfluid helium.

The goal of the present experiments is the investigation of foreign atoms and molecules in helium quantum crystals, focusing on their structure, their formation, and their light-induced dynamics. Liquid and solid helium play an outstanding role in modern physics as they show their quantum nature on a macroscopic scale. Alkali-metal atoms and molecules can be implanted by laser ablation into a helium crystal, where they can reside for many hours. The softness of the quantum crystal allows the implanted species (called defects) to conserve their symmetry to a large extent. The defects in their ground state strongly repel surrounding helium atoms, thereby forming so-called atomic/molecular bubbles in condensed helium. The interaction of excited atoms with helium is highly anisotropic and can result in the formation of bound quasi-molecular complexes, so-called exciplexes. We use spectroscopic and time resolved studies of laser induced fluorescence to investigate the structure and the interactions of the different types of defects with the surrounding helium.