Knowledge Bank

We use infrared spectroscopy to observe the quantum dynamics of molecular hydrogen trapped within the interstitial sites of the $C60$ lattice. These spectra form the first observation of infrared activity induced by the $C60$ intermolecular potential. They also reveal the first evidence for distinct side-bands (at +/- $127cm-1$) associated with the translational motion of the trapped species. On exposure of $C60$ to high pressure $H2$ the induced modes grow in intensity over the course of a few hours as the $H2$ slowly diffuses into the octahedral interstitial sites. Despite the fact that the fundamental $H2$ vibrational mode occurs highly red-shifted ($54cm-1$) relative to the gas phase the location of the rotatioanal side-bands indicates almost complete rotational freedom of the trapped $H2$. Most surprisingly the spectra contain weak central zero-phonon modes which should not be activated by a potential with true octahedral symmetry. These zero-phonon modes show a sudden decrease in intensity on cooling through the $C60$ orientational phase transition at 260 K. We speculate that this may be the result of freezing out of a quasi fluid like surface layer within the $C60$ lattice. Work supported by Research Corporation Grant # CC5240