Chemisorption of hexafluoroazomethane on Si (111) was studied with synchrotron photoemission as a model system for studies of halocarbon-based etching. C 1S, N 1s, and F 1s core level spectra were collected for various room temperature exposures of clean silicon to hexafluoroazomethane, and after mild annealing. The initial reaction of hexafluoroazomethane with silicon is shown to be quite complex, resulting in the formation of many adsorbed dissociation products. Annealing resulted in the desorption and decomposition of these species. These results demonstrate the buildup of a passivating carbon layer on the silicon samples, which may be responsible for the inability of CF x radicals to etch silicon effectively. © 1988 American Institute of Physics.