Neutron diffraction, elastic

Elastic neutron diffraction was first performed (analyzer in Fig. 1 set to zero energy transfer) to establish the structure of the monolayer at low temperature. Three Bragg reflections were observed which could be indexed by a triangular lattice having a nearest-neighbor distance about 10% smaller than required for a 3 X /3 R30° commensurate structure (every third carbon hexagon in the graphite basal plane occupied). 

The calculations with empirical potentials of the structure and dynamics of paraffin molecules adsorbed on graphite are presently being extended (27) to include the intermolecular interaction. This work together with elastic neutron diffraction experiments on deuterated paraffin films (see Sec. Ill) should provide a sounder basis for interpreting the inelastic neutron vibrational spectra. 

III. Determination of the Orientation of Adsorbed Molecules by Elastic Neutron Diffraction... 

Molecular orientational order in adsorbed monolayers can be inferred indirectly from elastic neutron diffraction experiments if it results in a structural phase transition which alters the translational symmetry of the 2D lattice. In such cases, Bragg reflections appear which are not present in the orientationally disordered state. Experiments of this type have inferred orientational order in monolayers of oxygen (41) and nitrogen (42) adsorbed on graphite. However, these experiments have not observed a sufficient number of Bragg reflections to determine the molecular orientation by comparing relative Bragg peak intensities with a model structure factor. 

Figure 10. Elastic neutron diffraction pattern from a 0.4-layer nitric oxide film adsorbed on a recompressed graphite powder Papyex (431. Background scattering from the substrate has been subtracted. The molecules are assumed to be lying down as shown in the unit cell (inset). The arrows indicate the position and relative intensities of the Bragg peaks calculated for this structure.

Figure 11. (a) Elastic neutron diffraction pattern from a 0.8-layer butane film... 

Hutton J, Nehnes RJ (1981) High-resolution studies of cubic perovskites by elastic neutron diffraction II SrTiOs, KMnFs, RbCaFa and CsPbCla. J Phys C Solid State Phys 14 1713-1736... 

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