Extended-x-ray-absorption-fine-structure (EXAFS) measurements have been made on 0.6- and 0.9- monolayer samples of ${\mathrm{Br}}_2$ adsorbed on Grafoil, a form of graphite, and on an intercalated sample between 100 and 293 K. For both of the adsorbed samples the ${\mathrm{Br}}_2$ 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 Å to accommodate part of the lattice mismatch. The average Br-C distance is 2.9 Å. 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 ${\mathrm{Br}}_2$-${\mathrm{Br}}_2$ interactions. In the intercalated sample the average Br-C distance decreases to 2.5 Å, 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.

• Received 12 December 1977

DOI:https://doi.org/10.1103/PhysRevB.17.4069