Lead


Lay summary
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.

Atomic bubbles
The spectroscopy of atomic bubbles in liquid and isotropic solid helium is well established. We extend these studies to the anisotropic (uniaxial) helium crystals, in where the bubbles loose their spherical shapes. This results in specific perturbations of the electronic structure of the defect, which can be measured with high precision.

Exciplexes
Alkali metal-helium exciplexes are very interesting objects in theoretical quantum chemistry as they can be modeled by modern computational methods.Solid helium is particularly well suited for the production and investigation of such weakly bound complexes.

Dimers
Alkali dimers play an outstanding role in the photoassociation of trapped ultracold atoms. In solid helium dimers form both bubble and exciplex states, which have not been studied before. Our experiments open a new line of research. We found that the photodissociation of dimers implanted in solid helium produces one excited and one ground state atom, which share a common bubble. This represents a novel quantum object with interesting properties, whose dynamics and structure can be studied via the fluorescence of its dissociation products: atomic bubbles, exciplexes, and long-range diatomic complexes.