I. Electron Spin Resonance of 87Rb Monomers

I. Introduction

Superfluid Helium Nanodroplets (He_N) are used as the perfect matrix for single atoms and small clusters because of their unique properties. Electron spin resonance (ESR) spectroscopy on He_N 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 He_N leads to an increase of the Fermi contact interaction and hence to a droplet size dependent change of the hyperfine constant a_HFS. 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 He_N 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 He_N [2] and a detailed analysis of the Rb-He_N system are presented.

Setup

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).