Molecular beam scattering studies on Si(111)-7 x 7, Si(111)-H(1 x 1), and Si(111)-D(1 x 1) surfaces

This thesis describes experimental results on clean Si(111)-7x7, H-atom and D-atom induced Si(111)-1x1 surfaces using molecular beam scattering with beam energies between 17 and 100 meV and beam species He, H₂, and mixture beams of He:H₂ and He:Ar. Evidence of the unavailability of the rotational ground state ('zero point') energy of ortho-H₂ is presented. The amount of energy converted to the kinetic energy of a beam is measured, especially the efficiency of the conversion of the rotational energy of H₂ to translational energy during the expansion. Helium atom scattering (HAS) experimental results on clean Si(111)-7x7 surfaces are presented for temperatures near 1130K at which there is a transition to a '1x1' surface. Especially notable is the specular peak height jump during the transition, which is observed as a decrease in other HAS experiments on this surface. A simple model shows that a decrease of the top bilayer height of the surface by 25% after the structural transition reconciles the different experimental results including similar ones on Ge(111) surfaces. Evidence for a 6-fold rotational symmetry of the Si(111) surface during the structural transformation, induced by H-atom adsorption, from the 3-fold symmetry of the DAS Si(111)-7x7 to the Si(111)-1x1 surface is presented. It has been found that the incident beam(probe) disturbs the system being measured as in the striking case of the H₂ beam on the D-atom covered surface where no diffraction peak is observed while there are diffraction peaks on the H-atom covered Si(111)-1x1 surface. The changes of the diffraction peak widths are measured as they vary with the incident beam energy and the masses of the outermost atoms on the surface.