Br₂ Induced Step Faceting of a Cu Surface

Dissociative adsorption of Br₂ on Cu(100) terraces results in a c(2x2) structure in which every other four-fold hollow surface site is occupied.  As a result of this structure, the close-packed directions of the adlayer are rotated 45^o with respect to those of the Cu substrate.  This causes the favored step direction to rotate by 45^o to align parallel to the close-packed direction of the adlayer.   This is readily seen in the three-dimensional rendering of an STM image of a vicinal Cu(100) surface shown above where the raised bumps correspond to the adsorbed Br atoms.