Deep inelastic neutron scattering of ${\mathrm{H}}_2$ in single-walled carbon nanotubes

We report deep inelastic neutron scattering studies of ${\mathrm{H}}_2$ adsorbed on single-walled carbon nanotubes. The kinetic energy per molecule $(77\mathrm{}\mathrm{K}\pm 8\mathrm{K})$ is consistent with the bulk value, but the observed and calculated rotational spectra differ from those of bulk ${\mathrm{H}}_2.$ These findings suggest an orientation-dependent interaction between the ${\mathrm{H}}_2$ and the carbon nanotubes and indicate that the hydrogen may be adsorbed in the interstitial channels or in the external grooves of the nanotubes. We present model calculations which indicate that the orientational potential is large.