Modelling of diffusion from equilibrium diffraction fluctuations in ordered phases

Measurements of the collective diffusion coefficient Dc at equilibrium are difficult because they are based on monitoring low amplitude concentration fluctuations generated spontaneously, that are difficult to measure experimentally. A new experimental method has been recently used to measure time-dependent correlation functions from the diffraction intensity fluctuations and was applied to measure thermal step fluctuations. The method has not been applied yet to measure superstructure intensity fluctuations in surface overlayers and to extract Dc. With Monte Carlo simulations we study equilibrium fluctuations in Ising lattice gas models with nearest neighbour attractive and repulsive interactions. The extracted diffusion coefficients are compared to the ones obtained from equilibrium methods. The new results are in good agreement with the results from the other methods, i.e., Dc decreases monotonically with coverage θ for attractive interactions and increases monotonically with θ for repulsive interactions. Even the absolute value of Dc agrees well with the results obtained with the probe area method. These results confirm that this diffraction based method is a novel, reliable way to measure Dc especially within the ordered region of the phase diagram when the superstructure spot has large intensity.