Adlayer-induced LEED beams near order–disorder transitions

We present a Monte Carlo study of the adlayer-induced LEED beams in a representative order–disorder transition. Calculations of the widths of fractional order beams indicate that, as seen in recent experimental work on the O/Ni(111) system, these widths rise sharply and suddenly above Tc. Furthermore the slope of this rise provides a sensitive probe of the adatom-adatom interactions. The increase in the width results from the existence of ordered domains above Tc, and has not been previously discussed since prior calculations have concentrated on beam intensities. We further derive a functional relation between the window approach used in simulations of order–disorder transitions and the instrument response function approach of the experimentalist, to detail the effect of the LEED apparatus on the profile of the extra beams.