The role of diffraction in electron-stimulated desorption (DESD) is demonstrated experimentally and described theoretically. Specifically, initial state effects in DESD of Cl ⁺ from Si(111)-(1 × 1):Cl and Si(111)-(7 × 7):Cl are examined and a theoretical treatment that includes spherical-wave effects and multiple scattering of low-energy incident electrons is presented. Although contributions from complicated defect configurations such as SiCl₂ and SiCl₃ cannot be ruled out, comparison of the experimental data with theory indicates that Cl ⁺ desorption from Si(111)-(1 × 1):Cl and Si(111)-(7 × 7):Cl surfaces may be dominated by monochloride terminal sites. The initial states probably contain significant Si 3s and/or Si–Cl σ-bonding character. In the Si(111)-(7 × 7):Cl case, these excitations favor a propensity for Cl ⁺ desorption from the unfaulted, rather than faulted, zones of the 7 × 7 reconstructed rest atom area. This propensity may be related to increased screening and hole localization in the Si–Si backbonds within the faulted region. Finally, introducing Debye–Waller factors into each scattering path accounts for much of the experimentally observed DESD width broadening at room temperature.