Quantum excitation spectrum of hydrogen adsorbed in nanoporous carbons observed by inelastic neutron scattering

Inelastic neutron scattering spectra have been collected over a wide range of momentum transfer from H₂ adsorbed in several high-porosity carbon substrates. We show theoretical spectra which consider the relationship between rotational and translational transitions in the highly anisotropic adsorption environment, proving that different rotational excitations contain different amount of recoil broadening and motivating a new analysis method which considers both types of transitions at once. Spectra for most of the samples, including two activated carbons, are very similar to one another, supporting models of nanoporous carbons which are quite similar on the sub-nanometer scale. The exception is the low-energy side of the rotational peak, indicating important differences in the initial distribution of motion. We also find more subtle differences in the spectra which may be linked to differences in sample heterogeneity and surface rugosity. One sample does have a very different spectrum, which is not explained by standard models of this system. We also observe a significantly reduced effective mass in the spectrum of recoil transitions and evidence of coupling of rotational and translational motion resulting from periodic variations in orientation of the rotational states.