An improved reliability factor for quantitative low-energy electron diffraction

Quantitative low-energy electron diffraction (LEEDI(V) or LEEDI(E)), which evaluates the diffraction intensitiesIas a function of the electron energy, is a versatile technique for the study of surface structures. The technique is based on optimizing the agreement between experimental and calculated intensities. Today, the most commonly used measure of agreement is Pendry'sRfactorRP. WhileRPhas many advantages it also has severe shortcomings, as it is a noisy target function for optimization and very sensitive to small offsets of the intensity. Furthermore,RP=0, which is meant to imply perfect agreement between twoI(E)curves, can also be achieved by qualitatively very different curves. We present a modifiedRfactorRS, which can be used as a direct replacement forRP, but avoids these shortcomings. We also demonstrate thatRSis as good asRPor better in steering the optimization to the correct result in the case of imperfections in the experimental data, while another commonRfactor,RZJ(suggested by Zanazzi and Jona) is worse in this respect.