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.

Acknowledgments

Most of the experimental work described herein was performed at the University of North Carolina at Chapel Hill in the group of Roger Miller, who died on 6 November 2005. The authors are very grateful for the opportunity to work with him and for the stimulating environment and support he provided to his group.

J. M. M. thanks Tom Baer for supporting him during the final stage of his graduate work. We thank Ad van der Avoird for providing the potential energy surfaces in figure 3. We wish to acknowledge detailed feedback by one referee.

Financial support by NSF and AFOSR is acknowledged. J. K. gratefully acknowledges a Feodor Lynen fellowship of the Alexander von Humboldt Foundation and J. M. M. a scholarship by the Max Planck Society.

Notes

† It is interesting to note that these studies–on systems which are quite similar to the X-HY systems described in section 4.1–were performed at the Kaiser Wilhelm-Institut in Berlin-Dahlem, the predecessor of the Fritz Haber-Institut where this review was written.

‡ More generally the change of dispersion and repulsion must cancel, i.e. for some coordinate system R: .

† Commercial atom sources provide so-called clean atomic beams, but for the purposes of the experiments described here the atom-content is still too low. Initial experiments by ourselves to use microwave discharge sources were not successful due to the necessary low gas flow and pressure, in order not to destroy the helium droplet beam by a gas-jet from the discharge source.

† It was recently proposed that similar effects should also be obtainable in atomic Bose–Einstein condensates, where ionic impurities could lead to the formation of mesoscopic molecular ions 240.

† In a similar approach groups from the field of ultracold atomic physics and helium droplet spectroscopy have successfully collaborated to obtain information on the PES and spectroscopy of mixed alkali dimers 256.