Neutron scattering study of H2 adsorption in single-walled carbon nanotubes

${\mathrm{H}}_{\mathrm{2}}$ adsorption in single-walled carbon nanotubes (SWCNTs) has been studied with quasielastic and inelastic neutron scattering. At 80 K, under a ${\mathrm{H}}_{\mathrm{2}}$ pressure of 110 atm, ${\mathrm{H}}_{\mathrm{2}}$ molecules gradually condense in the SWCNT sample. After pumping out at 25 K and 20 mTorr, the remaining ${\mathrm{H}}_{\mathrm{2}}$ molecules show a quantum rotational transition at $\text{14.5±0.1\hspace{0.05em}}\mathrm{meV},$ with a peak width that increases linearly with increasing temperature from 4.2 to 35 K. The ${\mathrm{H}}_{\mathrm{2}}$ molecules remain in the sample up to 65 K and then start to desorb with increasing temperature. A broad inelastic scattering spectrum from the adsorbed hydrogen is also observed. The time scale of the dynamics is longer than 15 ps even at 200 K. Our results imply that hydrogen molecules are physisorbed in the interstitial tunnels of the SWCNT bundles.