CURVE SYNTHESIS AND OPTIMIZATION PROCEDURES FOR X-RAY PHOTOELECTRON-SPECTROSCOPY

The selection of functions to represent XPS lineshapes is discussed. An improved Lorentzian/Gaussian product function is proposed for fitting symmetrical XPS peaks, and a further empirical modification is shown to represent asymmetric (metallic) XPS peak profiles very closely. One additional parameter sets the asymmetry at half-height, while a second governs the tail profile. An approach to curve synthesis that rapidly yields good first approximations to complex multicomponent spectra is described. Previous fits to model systems may be incorporated, and background intensity differences between the high- and low-energy sides of each model peak may be retained in the fit. A simple least-squares optimization procedure is described in which divergence is impossible but which is still sufficiently fast for microcomputer implementation. Physically desirable parameter constraints can be applied, and excessively large parameter changes are prevented, even for small components of the fit. The relative precisions of the computed parameters also are estimated.