Extended-x-ray-absorption-fine-structure study of the Br2-graphite system

Extended-x-ray-absorption-fine-structure (EXAFS) measurements have been made on 0.6- and 0.9- monolayer samples of Br2 adsorbed on Grafoil, a form of graphite, and on an intercalated sample between 100 and 293 K. For both of the adsorbed samples the Br2 molecule is found to lie parallel to the basal-plane surface with each atom aligned as well as it can be above adjacent hexagonal sites. The Br-Br distance increases about 0.03 to accommodate part of the lattice mismatch. The average Br-C distance is 2.9 . Both of these coverages seem to be in the two-dimensional liquid phase seen in low-energy-electron- diffraction measurements, although analysis of the Br-Br internuclear vibrational amplitude suggests increased ordering for the 0.9-monolayer sample as the temperature is lowered. These two coverages are distinctly different from results previously reported for 0.2 monolayer, demonstrating the importance of Br2-Br2 interactions. In the intercalated sample the average Br-C distance decreases to 2.5 , and the bromine seems to be mainly molecular with the Br-Br distance increasing to match the periodicity of the graphite lattice. There is also evidence that the intercalated sample is a mixture of two phases. Finally, the amplitude of the Br-Br EXAFS is found to exhibit puzzling deviations from that of the vapor.