Reflectivity spectra as absorption resonant spectra: is it correct?

Approximate expressions for X-ray resonant and Mössbauer reflectivity in the total external reflection region are developed for the limiting cases of a semi-infinite mirror with a small resonant addition to the total susceptibility and for the case of an ultrathin resonant layer. It is shown that in this region the reflectivity can depend linearly on the imaginary part of the refraction index; therefore in these cases the consideration of reflectivity spectra (R-spectra) as absorption resonant spectra, taken up in several experimental studies, can be justified. However, several effects producing dispersive distortions of the R-spectrum shape, even for very small grazing angles, have been found. It has been shown that dispersive corrections to the R-spectrum shape are mostly necessary if the non-resonant absorption is relatively large. Model calculations demonstrate that the quantitative spectroscopic information extracted from R-spectra using the software developed for absorption spectra can be inaccurate.