I. Introduction
Superfluid Helium Nanodroplets (HeN) are used as the perfect matrix for single atoms and small clusters because of their unique properties. Electron spin resonance (ESR) spectroscopy on HeN is a promising tool to study the environment of radicals at very low temperatures (0.38K), which could recently be demonstrated for 85Rb doped droplets [1].
Electronic perturbation of the alkali-metal atom induced by the HeN leads to an increase of the Fermi contact interaction and hence to a droplet size dependent change of the hyperfine constant aHFS. This opens the opportunity to use Rubidium as a spin label for ESR-silent species with high polarizabilities, dipole moments, and/or nuclear spins, which are embedded inside the HeN cryostat.
Because of its larger hyperfine coupling, 87Rb is more sensitive to additional influences due to the embedded species, such as a change of Van der Waals interaction. The first ESR spectra of single 87Rb atoms on HeN [2] and a detailed analysis of the Rb-HeN system are presented.
Setup
The Helium Nanodroplets are produced in a supersonic jet expansion. After single Rubidium atom pickup, the valence electron spins are aligned parallel (↑) or anti-parallel (↓) to the external magnetic field B0. A net electron spin polarization is established through optical pumping.
By applying a resonant microwave field spin flips are induced in resonance mode. These ESR transitions are detected indirectly by means of optically detected magnetic resonance (ODMR).