The adsorption of Cl 2 on Cu{111} at 300 K has been studied using shadow-cone-enhanced secondary ion mass spectrometry (SIMS). The system has been investigated for chlorine coverages ranging from 0.08 to 0.33 monolayer (ML), including the Cu{111}-(√3 x √3)R30°-Cl surface. The secondary Cu + ion intensity has been measured as a function of the incidence angle of the primary ion beam. The enhanced intensity features in the spectra are compared with results from a two-body interaction calculation that uses the Moliere approximation to the Thomas-Fermi potential. A chlorine-copper interlayer spacing of 1.87 ± 0.04 A is measured between the coverages of 0.17 and 0.33 ML. This value corresponds to a chlorine-copper bond length of 2.38 ± 0.04 A. At 0.08 ML, the chlorine-copper bond length is expanded to 2.48 ± 0.04 A. These results suggest that the chlorine-copper bond is more ionic in the low coverage limit.