Diffusive motions in 2D phases of ethane adsorbed on graphite

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Abstract
The diffusive rotational and translational mobility of a simple rod-like molecule, i.e. ethane (C2H6) adsorbed on graphite (0001), has been studied by quasi-elastic neutron scattering, between 5 and 122 K. The motion of the molecules packed in two typical two-dimensional (2D) solids has been investigated One solid (S1) has a herring-bone structure with molecules lying down on the graphite substrate. The second solid (S 3) of higher density is made of molecules standing up on the adsorbing surface. With increasing temperature, a rotational transition is observed, between the rotationally ordered solids and 2D crystals where the C2 H6 molecules exhibit uniaxial rotations about the C-C axis. The characteristic times for these rotations have been measured. At higher temperatures, the S1 and S3 solids melt The molecules in the higher density phase keep their uniaxial rotational motion and in addition exhibit a Brownian diffusion with diffusion coefficient ranging from 2 to 14 x 10-6 cm2. s-1 at 87 and 122 K respectively. For the S1 melted solid, the rotational motion becomes isotropic and the diffusion coefficient ranges from 0.4 x 10-6 Cm2.s-1 at 71.4 K to 50 x 10-6 cm2 . s-1 at 122 K. The activation energy of translational diffusion is about 1.4 kcal. mole-1. These results are compared with the mobility of C2H6 in its bulk phases