Diffusion and melting of adsorbed monolayers by Mössbauer spectroscopy

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Abstract
Mössbauer spectroscopy is applied to the study of dynamical properties of adsorbed monolayers with special attention to changes in resonant absorption cross section caused by diffusion along the surface. Specific systems studied were films of butadiene iron tricarbonyl on exfoliated graphite (Papyex) at several coverages, from 130 K to room temperature. The absorption spectra of the Fe57 constituent were measured in the direction parallel to the average film plane and normal to it. In the parallel direction, the temperature dependence of absorption is normal at low temperature, but decreases abruptly to undetectable levels near 150 K, indicating first order phase changes to a fluid phase. The results are analysed in terms of a detailed theory of diffusive broadening in surface films. Effects of crystallite orientation are treated, with specific reference to the partial alignment in Papyex and Grafoil