NMR STUDIES OF STRUCTURE AND DYNAMICS OF PHYSICALLY ADSORBED LAYERS ON UNIFORM SOLID SURFACES

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Abstract
R6sum6. - Les temps de relaxation spin rkseau des protons de benzkne adsorb6 sur graphite microcristallin dans une monocouche ont Btk Btudites. Le modkle subskquent rend possible d'interprkter les rBsultats expBrimentaux. Au-dessous de - 150 "C les molBcules sont orientkes perpendiculairement B la surface et localiskes sur des emplacements dans une structure hexagonale. Les molBcules exkcutent des sauts de rkorientation autour de leurs axes de symktrie C, B la frkquence 2,3 x 1013 exp(- 1 060/T) s-I. - From proton spin lattice relaxation time measurements a model for the structure and microdynamics of a benzene monolayer on graphitized carbon black is developed. The essential results are that at temperatures below - 150 OC benzene molecules are absorbed at definite sites of the graphite (0001) planes in ordered structure of hexagonal symmetry with the molecular planes oriented perpendicularly to the surface. The molecules perform reorientational jumps about diad molecular symmetry axes at a rate of 2.3 x loJ3 exp(- 1 060/T) s-'. 1. Introduction. - In recent years the study of physisorbed layers on graphite has been promoted by the application of modern surface analysis techniques such as LEED and AES (I-31. Whereas a rather clear-cut picture of the structure and phase transitions of layers of noble gases has been developed only little is known by such techniques about layers of physisorbed molecules (4). On the other hand, an exceedingly large experimental material based on classical calorimetric and adsorption isotherm studies using graphitized car- bon blacks as adsorbents may be found in the literature (5-71. In addition, several theoretical papers have appeared which are concerned with the calculation of molecule-graphite surface potential energies (7,8). However, the results of the above mentioned classical studies have up to date not found a satisfactory interpretation in terms of structural and dynamical models for the adsorption layers because of lack of knowledge on the nature of the adsorption forces, on the vibrational modes and

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