Diffraction studies of the multilayer structure of simple physisorbed films
Elastic neutron diffraction has been used to study the structure of N[sub 2] films adsorbed on the (0001) surfaces of an exfoliated graphite substrate with coverages from 1.0 to the coexistence of bulk N[sub 2]-[alpha] and the film at temperatures below 30 K. At monolayer, the investigation is focused at the molecular orientations while the film structure evolves from the in-plane commensurate phase at [theta] = 1.0 to the out-of-plane triangular incommensurate phase at [theta] = 1.67. At higher coverages, the bulk N[sub 2]-[alpha] starts to nucleate at [theta] = 3.70 after the formation of a bilayer physisorbed film. This incomplete wetting occurs due to the distortion of the lattice structure and the mismatch of the latter constant between the film and the bulk crystal. The first order phase transition occurring at 23.4 K is interpreted as melting of the second layer rather than a layering transition proposed by Zhang et al. New phase diagrams which are consistent with different experimental observations of N[sub 2]/graphite at temperatures below 30 K are given in the thesis. An ultra-high vacuum chamber developed for structural analysis of adsorbed films and single-crystal surfaces using synchrotron x-ray diffraction is described. It is particularly suited for investigations of physisorbed and other weakly bound films. A low-energy electron diffraction (LEED)/Auger spectroscopy system with a position-sensitive detector enables in situ characterization of the film and substrate while the sample is located at the x-ray scattering position. A closed-cycle He refrigerator and electron bombardment heater provide controlled substrate temperatures from 30 K to 1300 K. A new procedure to calculate the circle angles of the diffractometer for various grazing-angle scans is developed and described. Finally, to demonstrate the performance of the instrument, the authors present some preliminary results of a study of Xe physisorbed on the Ag(111) surface.
- Research Organization:
- Missouri Univ., Columbia, MO (United States)
- OSTI ID:
- 7197034
- Resource Relation:
- Other Information: Thesis (Ph.D.)
- Country of Publication:
- United States
- Language:
- English
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