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Properties of H₂, Ar, and Ne clusters in superfluid ⁴He nanodroplets : towards a search for superfluidity in large supercooled H₂ clusters Nakahara, Hiroko

Abstract

The ultimate goal of this research project is to develop an experiment to probe for superfluity in large clusters of molecular hydrogen in ultra-cold helium-4 nanodroplets. Superfluidity has now been observed in a wide variety of systems and hydrogen is a good candidate to exhibit this macroscopic quantum phenomenon in a molecular system. In this thesis, two major advances were made enroute to the eventual search for superfluidity in bulk clusters of molecular hydrogen. 1. In the first advance, the fluidity of supercooled molecular H₂ was investigated in helium-4 nanodroples (~ 10⁵ atoms) at 0.38 K. To clearly demonstrate that the H₂ clusters are fluxional, or fluid-like, separate studies of argon and neon clusters were also made for comparison. To probe the behavior of the clusters, a single tetracene probe molecule was also inserted into the droplet and the laser induced fluorescence (LIF) from the tetracene was studied as a function of the cluster size and the pickup method. In the prior pickup method, the cluster species is added to the ⁴He droplet before to the probe molecule and in the post pickup method, the tetracene is added and then the cluster species is added. Due to the difference in the pickup order, the configuration of the probe molecule and the cluster species can differ. The observed spectral shift of tetracene LIF in the presence of the cluster species was studied for both pickup methods. For Ar and Ne clusters, the spectral shifts from the prior and post pickup methods show clear differences. This observation suggests that for prior pickup, the tetracene molecule attaches to the surface of the cluster and does not penetrate into the centre of the cluster and we conclude that the Ar and Ne clusters are not fluid-like in the helium droplets. For para-hydrogen and normal-hydrogen the LIF spectra of tetracene are independent of pickup order and we conclude that the supercooled H₂ clusters remain fluid-like at 0.38 K. 2. The second advance made in this thesis was to configure the droplet apparatus to study the rotational states of probe molecules in ⁴He droplets doped with H₂ clusters. The rotational states are studied by a combination of infrared and mass spectroscopy. Methane is the probe molecule used and when introduced into the ⁴He droplet it is surrounded by the H₂ cluster. If the surrounding H₂ liquid is superfluid, the methane rotates freely with a low moment of inertia. Conversely, if the H₂ remains a normal fluid, the dopant molecule drags hydrogen molecules along as it rotates and has a much larger moment of inertia. Rotationally resolved infrared spectroscopy of the methane gives clear information about the state of the surrounding supercooled liquid H₂. As a first step, the v3 vibrational mode of bare methane in ⁴He droplets was studied. The R(0) transition of the v3 stretching mode of methane was partially observed and found to be consistent with the R(0) peak for CH⁴-doped ⁴He droplet systems previously measured by the Miller group [1].

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