Determination of TiO2(110) surface relaxation by variable polarization photoelectron diffraction

A new method to determine the surface relaxation by means of photoelectron diffraction (PED) from an s-core level has been applied to the (1x1)-TiO2(110) surface. Complete 2 pi solid angle FED patterns for both oxygen Is and titanium 3s core level peak intensities have been measured at the ALOISA beamline (Elettra Synchrotron, Italy) in two different photon polarization/electron detection geometrical configurations: one with the photon polarization vector parallel to the surface normal (TM polarization) and the other with the photon polarization vector perpendicular to the surface normal (TE polarization). In the latter case the FED sensitivity to the vertical spacing is enhanced. Because forward scattering is suppressed on the node of the primary wave along the surface normal, the relative weight of the distance-sensitive higher order interference fringes is enhanced. The opposite occurs for the TM polarization, which enhances the FED sensitivity to lateral spacing at grazing emission angles. The experimental FED patterns are compared with the theoretical simulations calculated for the relaxed and unrelaxed TiO2(110) surface. The results prove the feasibility of the method and the adequacy of the FED technique to the studies of relaxation phenomena at crystal surfaces.