By comparing the C 1s photoemission intensities from a clean diamond (111) surface to those obtained from a fluorine-covered surface, we conclude that a single (111) layer of atoms participates in the diamond 2×1 surface reconstruction. The data interpretation presented does not rely on assuming an electron mean free path, but, in fact provides an independent measurement of its value. For clean diamond (111) a surface core-level peak is observed at 0.80±0.05 eV lower binding energy than the bulk-C 1s peak at 285.0 eV below EF. After fluorine exposure a single chemically shifted carbon peak is observed at 1.85±0.05 eV higher binding energy than the bulk peak, indicating the presence of CF units at the surface. © 1986 The American Physical Society.