Reflectivity spectra as absorption ones -- is it correct ?

Approximate expressions for X-ray resonant and M\"ossbauer reflectivity in the total external reflection region are developed for the limiting cases of a semiinfinite 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 really show the linear dependence on the imaginary part of resonant susceptibility, therefore in these cases the consideration of reflectivity spectra (R-spectra) as absorption resonant spectra, undertaken in several experimental works, can be justified. However, we have selected several effects producing essential dispersive distortions of the R-spectrum shape even for very small grazing angles. It has been shown that the dispersive corrections to the R-spectrum shape are mostly essential if the nonresonant absorption is relatively large. Model calculations demonstrate that the quantitative spectroscopic information extracted from R-spectra by using the software developed for absorption spectra can be inaccurate.