The structure of ammonia overlayers physisorbed onto the surface of single crystal graphite, determined by means of atomic beam diffraction

Abstract

The structure of NH3 monolayers and submonolayers physisorbed on graphite C(0001) has been determined by means of low energy helium atom beam diffraction experiments. The measured diffraction pattern is found to consist of diffraction ‘‘rings,’’ indicating a large degree of orientational disorder among the adsorbed domains. We observe up to fourth order diffraction and periodicities which can be generated from a rectangular 3.53×6.11 Å unit cell or by a 7.06×7.06 Å hexagonal mesh. Both of these possibilities are characteristic of a 3.53 Å nearest neighbor separation. These findings suggest that, because of the relatively strong molecule–molecule interactions, the degree of order within a given domain is relatively high, while the domain’s orientational disorder may be due to the insensitivity of the adlayer to the corrugation and anisotropy of the graphite substrate. This behavior is atypical, since HCl, CH3F, CH3Cl, and CH3Br all form well organized monolayers which have a single well defined orientation with respect to the graphite substrate.