The use of resonant soft X-ray reflectivity (RSXRR) in s-polarization is presented with the aim to show how far it is possible to go in the understanding the evolution of the refractive index n ( E ) = 1 ? δ ( E ) ? i β ( E ) of a ultrathin polystyrene film when the RSXRR is measured through the C-edge. We evidence that a direct fit to the data provides a very good estimation of δ ( E ) and β ( E ) in a large range of energies. Nevertheless, at some specific energy close to C-edge we observe that it is not possible to obtain a satisfactory fit to the data though the same formalism is applied to calculate the reflectivity. We show that even though we take into account the energy resolution of the incident beam, we still end up with a poor fit at these energies. Incorporating the strong contribution of 2nd order photons appeared near C-edge we could not eliminate the discrepancy. Probably the data normalisations have some impacts on such discrepancies at some specific energies.
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