Neutron scattering study of H-2 adsorption in single-walled carbon nanotubes

H-2 adsorption in single-walled carbon nanotubes (SWCNTs) has been studied with quasielastic and inelastic neutron scattering. At 80 K, under a H-2 pr essure of 110 atm, H-2 molecules gradually condense in the SWCNT sample. Af ter pumping out at 25 K and 20 mTorr, the remaining H-2 molecules show a qu antum rotational transition at 14.5 +/-0.1 meV, with a peak width that incr eases linearly with increasing temperature from 4.2 to 35 K. The H-2 molecu les remain in the sample up to 65 K and then start to desorb with increasin g temperature. A broad inelastic scattering spectrum from the adsorbed hydr ogen 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. (C) 2001 American Institute of Physics.