In situ inelastic neutron scattering studies of the rotational and translational dynamics of molecular hydrogen adsorbed in single-wall carbon nanotubes (SWNTs)

• Published in: Carbon (New York) Vol. 43; no. 5; pp. 895 - 906
• Main Authors: Georgiev, P.A., Ross, D.K., De Monte, A., Montaretto-Marullo, U., Edwards, R.A.H., Ramirez-Cuesta, A.J., Adams, M.A., Colognesi, D.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 2005; Elsevier Science
• Subjects: Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Fullerenes and related materials; diamonds, graphite; Gas storage; Materials science; Nanotubes; Neutron scattering; Physics; Specific materials; Neutron scattering; Gas storage; Nanotubes; Gas solid adsorption; Singlewalled nanotube; Inelastic scattering; Hydrogen; In situ; Adsorbed state; Molecular dynamics; Carbon nanotubes; Carbon; Neutron diffusion
• ISSN: 0008-6223; 1873-3891
• DOI: 10.1016/j.carbon.2004.11.006

Inelastic neutron scattering (INS) spectra were measured in situ from progressively increased amounts of para-hydrogen physisorbed in bundles of single-walled carbon nanotubes at temperatures in the vicinity of 20 K. INS from the bound H 2 molecules consists of two distinct parts carrying complementary information. In the low energy and momentum transfer region, at about 14.5 meV we observe a sharp line corresponding to rotational transitions between the ground para- J = 0 state and the ground ortho- J = 1 state without change of the translational state of the molecular centre of mass (CoM). This we call the “bound” spectrum. At higher energy transfers, a series of broad peaks are observed, corresponding to rotational transitions between the para- J = 0 state and different ortho-states ( J = 1, 3, 5, … ,) shifted out in energy transfer by an amount equal to the CoM recoil energy. This we call the “recoil” spectrum. Both parts of each spectrum are analysed using the Young and Koppel model. From the “bound” spectrum we estimate the mean height of the barrier to rotation and the mean square displacements of the molecules accommodated at different adsorption sites. The “recoil” spectrum allows us to derive the mean translational kinetic energy of the adsorbed hydrogen as a function of the surface concentration.