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Hydrogen Molecules in Nanoscale Cavities: Quantum Dynamics, Inelastic Neutron Scattering Spectroscopy, and a New and Unexpected Rule for H2@C60

Zlatko Bačić
New York University

The behavior of hydrogen molecules inside nanoscale cavities of diverse host materials, e.g., fullerenes, carbon nanotubes, clathrate hydrates, and metal-organic frameworks, has received a great deal of attention in recent years. Much of the research has been driven by the potential which some of these systems have for hydrogen storage applications. In nanoscale confinement, the transnational motions of the caged molecules are quantized and strongly coupled to the molecular rotations, which are also quantized. I will review our rigorous quantum treatment of the intricate coupled translation-rotation (TR) dynamics of the caged H2/HD/D2, their dependence on the symmetry of the nanocavity, and the distinct spectroscopic signatures of the TR coupling that we have identified. These TR eigenstates are directly probed by the inelastic neutron scattering (INS) spectroscopy. Therefore, I will also present our recently developed methodology for accurate quantum simulation of the INS spectra of a hydrogen molecule in a nanocavity of an arbitrary shape, its implementation to H2/HD in clathrate hydrates, C60, and C70, and comparison with the measured INS spectra. A new and unexpected selection rule that we have derived for the INS spectroscopy of H2/HD in a near-spherical cage such as C60 will be highlighted. It explains why the INS transitions between certain TR eigenstates of H2/HD in C60 have zero intensity and do not appear in the spectra. Our theoretical predictions have been confirmed by the recently measured INS spectra of H2@C60, thus validating the selection rule - the first ever identified in INS spectroscopy.