Spectroscopy of free radicals and radical containing entrance-channel complexes in superfluid helium nanodroplets

Abstract

The spectroscopy of free radicals and of radical containing entrance-channel complexes embedded in superfluid helium nanodroplets is reviewed. The collection of dopants inside individual droplets represents a micro-canonical ensemble, and as such each droplet may be considered an isolated cryo-reactor. The unique properties of the droplets, namely their low temperature (0.4 K) and fast cooling rates (∼ 1016 K s−1), provide novel opportunities for the formation and high-resolution study of molecular complexes containing free radicals. Radical production methods are discussed in the light of their applicability for embedding radicals in helium droplets. The spectroscopic studies performed to date on molecular radicals and on entrance / exit-channel complexes of radicals with stable molecules are detailed. The observed complexes provide new information on the potential energy surfaces of several fundamental chemical reactions and on the intermolecular interactions present in open-shell systems. Prospects for further experiments of radicals embedded in helium droplets are discussed, especially the possibility of preparing, studying, and manipulating high-energy structures, as well as the possibility of using them in fundamental physics experiments.