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 T//c. 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 T//c, 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.