ESR in He nanodroplets to study magnetism and spin dynamics

Magnetic resonance experiments provide precise and detailed information about structure, magnetism, and dynamics of materials at the atomic level. Especially, electron spin reso-nance (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 mar-riage of these two powerful methods, and the very first ESR spectra of single alkali-metal atoms isolated in HeN were obtained. 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 uni-versal 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 inter-actions 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 conven-ient 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 fundamen-tal and technical interest. Finally, the method will be expanded to species with fast spin re- laxation, such as triplet state alkali-metal dimers. This research links atomic and molecular physics with areas of finite size condensed mat-ter, 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 dipole-dipole interac-tions.

Superfluid helium nanodroplets of about 5 nm diameter and 0.4 Kelvin temperature are a promising approach for the synthesis and investigation of exotic molecules or novel nanomaterials. The goal of this project was to investigate the influence of the quantum fluid on atoms and molecules in the ground and excited states. Especially, the dynamics induced by optical excitation of dopant particles is one of the most pertinent questions.The increasing popularity of helium nandroplets is based on the fact that the droplets cool dopants efficiently to the ground state in which the perturbation by the surrounding helium is often negligible. Electron spin resonance, a method that was developed in helium droplets in a former FWF project, is the method of choice to investigate this ground state perturbation. In this project the method could be considerably improved and a minimal, droplet size dependent compression of the valence electron wave function was measured with high precision.This perturbation is considerably stronger upon photo-excitation of the atoms. An expansion of the valence electron orbital leads to a much stronger repulsive interaction with the surrounding superfluid and triggers complex relaxation dynamics. A series of laser spectroscopic experiments performed in the course of this project on single chromium (Cr) and copper (Cu) atoms leads to the following picture: 1. Due to the repulsive interaction the atoms are ejected from the droplet. 2. During the ejection the excited atoms have to traverse the superfluid helium, which leads to electronic relaxation, often via dipole-forbidden transitions to metastable states. 3. The formation of Cr-He molecules is an important step in the relaxation paths. 4. Larger droplets result in more efficient relaxation and thus the population of lower states.These results are important for future studies on photoinduced relaxation experiments in helium droplets. For example, femtosecond time-resolved experiments on molecules of biological relevance such as nucleobases seem to be promising. The influence of additional molecules such as water on the relaxation dynamics can be studied in a controlled manner inside a helium droplet.