Processos de otimização na análise LEED e estudo da estrutura dos sistemas Ag(110) e Ag(111)/Sb

Abstract

This work is essentially composed of three studies concerned with the Low Energy Electron Difiraction (LEED) technique. In the first part we present the results obtained from the application of the fast simulated annealing (FSA) global search algorithm, for the first time, to the determination of a surface structure by LEED. The use of this algorithm in the determination of the structure of the Ag(111), Ag(110) and CdTe(110) surfaces allowed a characterization of the method. A scaling behaviour, that measures the eficiency of a search method as a function of the number of parameters to be varied, was obtained, indicating a favourable linear scaling (N1). In the second part of this work a study of the thermal expansion of the Ag(110) surface in the temperature range of 118 to 573 K is presented. The first three interlayer distances, as well as the Debye temperatures of the first two atomic layers were extracted from the LEED analysis. The observation of a linear expansion behaviour for the first two atomic layers, as well as an almost constant behaviour of the Debye temperatures seems to indicate negligible efiects of anharmonicity in the explored temperature range. Lastly, in the third part we present the results obtained from an analysis of the topography of the Ag(111)(2p3 fi 2p3)R30o Sb surface, where several models were proposed and tested. Although the results obtained did not allow a final conclusion, it was possible to exclude some classes of models and to suggest a `overlayer-substitutional' model as the most reasonable possibility.