Determination of low-energy electron diffraction beam profiles with application to ordering kinetics at theCu3Au(110) surface

The determination of the intensity profiles of low-energy electron diffraction (LEED) beams from data obtained by position-sensitive detection is described, and the observation of ordering kinetics at the ${\mathrm{Cu}}_3$Au(110) surface is reported as an example of the procedure. The beam profile is represented by a function of two parameters ${\mathit{q}}_1$ and ${\mathit{q}}_2$, where ${\mathit{q}}_1$ represents the width, and ${\mathit{q}}_2$ the shape, of the profile. The values of these parameters and of the peak intensity are reported as a function of ordering time t elapsed after a disordering anneal temperature ${\mathit{T}}_{\mathit{A}}$ above the surface compositional disordering transition temperature ${\mathit{T}}_1$, followed by quenching to a temperature ${\mathit{T}}_{\mathit{Q}}$ below ${\mathit{T}}_1$. Taking advantage of the circumstance ${\mathit{T}}_1$${\mathit{T}}_0$, where ${\mathit{T}}_0$ is the bulk disordering temperature, different initial states of disorder were selected depending on the value of ${\mathit{T}}_{\mathit{A}}$ relative to ${\mathit{T}}_0$: (a) ${\mathit{T}}_{\mathit{A}}$${\mathit{T}}_0$, ordered substrate; (b) ${\mathit{T}}_{\mathit{A}}$>${\mathit{T}}_0$, disordered substrate. On the basis of determinations of ${\mathit{q}}_1$(t) and ${\mathit{q}}_2$(t) for series of fixed values of ${\mathit{T}}_{\mathit{Q}}$, it is concluded that the ordering proceeds in two distinct stages, of which the first resembles nucleation of microclusters and the second the ‘‘annealing out’’ of domain walls. For initial state (b) evidence is presented that the first stage is an unprecedented case of two-dimensional compositional ordering.