Molecular rotation in doped superfluid clusters

Abstract

Experiments where impurities were incorporated into helium nanodroplets have shown that the impurity freely rotates, and this has been attributed to the superfluidity of the nanodroplet [1]. Results from experiments with smaller helium clusters suggest that the onset of superfluidity is linked to system size and bosonic exchange effects [2]. We have used path integral techniques to investigate these systems and predict their spectroscopic behaviour in the microwave and the infrared regions of the spectrum. We are particularly interested in observing the superfluid response in clusters where the helium atoms have been substituted with parahydrogen molecules. Molecular hydrogen has been suggested as a potential candidate for the observation of superfluid response but this substance crystallizes before reaching a temperature low enough for superfluidity to appear. We will show theoretical and experimental results of a molecular superfluid response at the nanoscale via the formation of doped hydrogen clusters with a carbon dioxide probe molecule [3]. Properties such as density distributions, spectroscopic features, and effective rotational inertia can be extracted from the simulations. We will show new results for the case of asymmetric top molecules embedded in superfluid para-hydrogen clusters. A perspective on the current challenges of the field will be presented. [1] Grebenev, Toennies, and Vilesov, Science 279, 2083 (1998); Toennies and Vilesov, Angew. Chem.-Int. Edit. 43, 2622-2648 (2004). [2] Tang, Xu, McKellar and Jäger, Science 297, 2030 (2002) [3] Li, Le Roy, Roy, and McKellar, Phys. Rev. Lett. 105, 133401 (2010)