Figure 5. Characteristic evaporation patterns for kink and central atoms. In red plots are intensity profiles of the atoms marked in blue, across black arrows, connecting the atom to the center of the pole, on their highest intensity image. a: Terrace atom evaporating “out of sequence.” b: Atom evaporating in sequence with its plane. c: Central atom evapor- ating in sequence, exhibiting similar intensity profile to the “out of sequence” terrace atom in (d). c,d: Can be distinguished by the intersection of the intensity and derivative plots along the black arrows (derivatives plotted in black).
In order to distinguish between these cases, it is necessary to examine not only their intensity profiles, but also the corre- sponding derivatives of these profiles. The derivatives are plotted by the black curves in Figures 5c and 5d, along with their respective intensity profiles in red on the same plot windows. It can be seen that an additional condition for the identification of an “out of sequence” kink atom, is that the rate of intensity change along the transition between the two planes will be sufficiently large. This condition is specifically illustrated by comparing intensity profiles and their
derivatives on the same figure as shown in Figures 5c and 5d. The derivative should change significantly enough within the transition region between the two planes so that it then crosses the intensity profile when plotted on the same axes. The algorithm therefore scans for such profiles and deriva- tives shapes across all atoms examined. Once a “cross-layer” event is identified, the atom will be assigned to the plane below the one under active consideration. Note that the “z” coordinate determined here represents the serial number of the crystallographic layer that the atoms
