Abstract

Helium nanodroplets formed in a supersonic expansion of high pressure gas from a low temperature source (10 to 20K) represent finite size superfluid samples with 0.4 K internal temperature. Using a helium backing pressure around 100 bar, nanodroplets of average size N=10⁴ atoms are generated. In a molecular beam apparatus, the droplets pass through a pick-up cell containing a gas at low pressure, where each droplet is doped with a statistical number of atoms or molecules. Most degrees of freedom of these dopants are immediately cooled to 0.4 K, and the released energy leads to evaporation of a corresponding number of helium atoms from the droplet. The cold environment can be used to cool individual molecules for a spectroscopic study of their lowest energy configuration, to produce weakly bound molecular complexes, or study cold collisions between different dopant species residing in the same droplet. In the analysis of condensed matter samples, electron spin resonance is a common experimental technique. ESR spectroscopy could become a versatile tool to investigate oligomers formed on He droplets after an examination of spin lifetimes and the development of a detection method for spin states. This paper reports the first magnetic study of any species in or on helium droplets as well as approximate relaxation times for electron spin states of potassium atoms and molecules on helium droplets.

© 2007 IEEE