The Assembly and Spectroscopic Interrogation of Hydrogen Clusters in Helium Droplets

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Andrey Vilesov of the University of Southern California is supported by the Experimental Physical Chemistry Program to continue his experimental studies of neat and doped helium and hydrogen quantum clusters. Experimental methods involve infrared pulsed laser depletion spectroscopy of molecules and molecular clusters in helium droplets, and CARS spectroscopy of neat hydrogen clusters. The ultimate goal of this research is to uncover how the properties of a superfluid evolve upon reducing the dimension of a system from bulk to small droplets, which may contain from several thousand to a few tens of helium atoms or hydrogen molecules. Spectroscopic studies will address the question of what the macroscopic concept of superfluid and non-superfluid fractions of liquid helium in the vicinity of a molecule means in terms of the quantum mechanical interaction of the molecule and its helium solvation shell. As well, new knowledge of the structure and quantum states of helium and hydrogen clusters and their interaction with molecules have the potential to make a valuable contribution to molecular spectroscopy, for which fluxional clusters present a considerable challenge at present.Superfluids are able to flow endlessly without friction. Hydrogen clusters, because they are made up of molecules, can be examined for superfluidity without the need for doping by other molecules as is required for atomic fluid helium droplets. This project aims to address a number of long-standing and intriguing issues, such as learning to detect superfluidity in microscopic droplets, discovering what it means for a cluster to exhibit superfluid behavior, and determining how many atoms/molecules are needed for a fluid to become superfluid.