I'm having some trouble understanding surface reconstruction diffraction patterns. Please see Figure 2 on the third page of the attached document. An in-depth knowledge of the subject matter isn't needed, I just need someone more familiar with reconstruction and diffraction than myself. I need help understanding two things: 1) In figure 2 (a) has the caption: "in addition to the 1x1 diffraction features, a weak 4x1 diffraction pattern consisting of three additional lines (marked by three arrows) within the 1x1 pattern is also visible." Why do three additional fractional visible lines indicate a 4x1 reconstruction pattern? 2) In figure 2 (b), please explain how the LEED image indicates a two-domain (1x4)/(4x1) reconstruction. Is there a way to "outline" the domains? I'd like to be able to identify them from the picture. I'd really appreciate any insight anyone can give. I'm very new to this stuff.