DETERMINATION OF LOW-ENERGY ELECTRON-DIFFRACTION BEAM PROFILES WITH APPLICATION TO ORDERING KINETICS AT THE CU3AU(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 Cu3Au(110) surface is reported as an example of the procedure. The beam profile is represented by a function of two parameters q1 and q2, where q1 represents the width, and q2 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 TA above the surface compositional disordering transition temperature T1, followed by quenching to a temperature TQ below T1. Taking advantage of the circumstance T1<T0, where T0 is the bulk disordering temperature, different initial states of disorder were selected depending on the value of TA relative to T0: (a) TA<T0, ordered substrate; (b) TA>T0, disordered substrate. On the basis of determinations of q1(t) and q2(t) for series of fixed values of TQ, 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. © 1990 The American Physical Society.