Magnetic resonance experiments provide precise and detailed information about structure, magnetism, and dynamics of materials at the atomic level. Especially, electron spin resonance (ESR) is highly suitable for magnetic studies, even in the case of ESR silent species with the application of spin labelling. Superfluid helium nanodroplets (HeN) are a flexible tool to study single atoms, molecules, or clusters in a cold (0.38 K) and isolated environment. Virtually any species can be doped into a HeN and tailored, often weakly bound complexes can be investigated. We have recently been able to demonstrate for the first time the marriage of these two powerful methods, and the very first ESR spectra of single alkali-metal atoms isolated in HeN were obtained [M. Koch, G. Auböck, C. Callegari, and W. E. Ernst. Coherent spin manipulation and ESR on superfluid helium nanodroplets. Physical Review Letters, 103(3):03530214, July 2009]. The unique environment of a HeN results in extremely sharp ESR lines and the minute perturbation by the droplet can be rationalized by an increase of the Fermi contact term (about +400 parts per million), which depends on the droplet size. Furthermore, ESR in HeN turned out to be a coherent process with long spin lifetimes.
The high sensitivity of ESR in HeN has opened the door for magnetic and spin dynamic investigations in a HeN. This project is aimed at the development of ESR in HeN into a universal tool to study magnetism and spin dynamics of single atoms, molecules, and clusters. In a first step, a surface-located rubidium (Rb) atom will be used as spin label for ESR silent species located inside the droplet. Information about van der Waals and dipole-dipole interactions between the surface Rb atom and the solvated particle of interest will be deduced from alterations of the well known Rb-HeN ESR spectrum, and the droplet size is a convenient handle to study these interactions as a function of distance. In a second step, single copper and chromium atoms will be investigated with ESR inside HeN. This is a first goal towards the magnetic investigation of metal clusters inside HeN, which is of great fundamental and technical interest. Finally, the method will be expanded to species with fast spin relaxation, such as triplet state alkali-metal dimers.
This research links atomic and molecular physics with areas of finite size condensed matter, magnetism in mesosopic systems, and single molecule reactions in chemical physics. Most notable, the possibility of adjusting the distance between two interacting particles offers fundamental new opportunities for the investigation, e.g., of magnetic dipoledipole interactions.