Infrared Spectroscopy of Hydrogen in Fullerite and MOF-5 Hosts

We present a novel use of diffuse reflectance infrared spectroscopy to study the quantum dynamics of hydrogen molecules trapped within a host material. This technique is particularly useful for the study of hydrogen storage materials since it provides detailed information about the intermolecular potential at the binding site. Because H$_{2}$ has no intrinsic infrared activity any observed features arise solely through interaction with the host material and as such are very sensitive to the symmetry of the binding site. The drawback is that the induced spectra are quite weak. However, a technique based on diffuse reflectance has been shown to produce a sufficiently large signal [1]. We have now constructed a cryogenic system that allows spectra to be obtained in this manner at pressures as high as 100 atm. and at temperatures as low as 10 K. Data will be presented for H$_{2}$ in both C$_{60} $ and MOF-5 showing a series of absorption features arising from the quantized vibrational, rotational, and translational motion of the trapped H$_{2}$. At the lowest temperature these peaks become quite sharp, FWHM less than 1 cm$^{-1}$, with a detailed fine structure arising from the H$_{2}$ host interactions. \\ 1. S. A. FitzGerald et al., Phys. Rev. B {\bf 65}, 140302. (2002)