Selective adsorption of ¹H₂ and ²H₂ on the (0001) graphite surface

Scattering of ¹H₂ and ²H₂ from a (0001) graphite surface at 100 K has been performed to study the laterally averaged molecule-surface interaction potential. The bound state spectrum has been determined by selective adsorption measurements. Energy levels at 1.4₆, 3.6₁, 7.9₆, 15.3₃, 26.4₃, 41.6₁ meV for ¹H₂ and at 1.9₃, 3.7₈, 6.3₇, 10.0, 15.4, 23.1 meV for ²H₂ were detected with an experimental accuracy of 0.25 meV. The two lowest levels for ²H₂ could not be observed in our experimental conditions. The experimental energy levels are in excellent agreement with those calculated for the potential V(z) = D[(1 + λ z p)^-2p - 2(1 + λ z p)^-p] with D = 51.7 ± 0.5 meV, p = 3.9 ± 0.2 and λ = 1.45 ± 0.02 A ̊^-1. A comparison with the theoretical model proposed by Crowell [1] to describe the interaction of non polar molecules with graphite has been carried out. Improvements to the Crowell model taking into account either higher order dispersion terms or the exponential behaviour of the repulsive interaction, are discussed.