I. Electron Spin Resonance of 87Rb Monomers

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

Setup of the ESR experiment

Rubidium pickup and detection on Helium nanodroplets

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).

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