Editing X-ray absorption fine structure (section)

==Atomic spectra==

The atomic X-ray absorption spectrum (XAS) of a [[core-level]] in an absorbing [[atom]]  is separated into states in the discrete part of the spectrum called "'''bounds final states'''" or "[[Rydberg state|Rydberg states]]" below the ionization potential (IP) and  '''"states in the continuum'''" part of the spectrum above the ionization potential due to excitations of the photoelectron in the vacuum. Above the IP the absorption cross section attenuates gradually with the X-ray energy. 
Following early experimental and theoretical works in the thirties,<ref name="fano1">{{cite journal | last=Fano | first=Ugo | title=Sullo spettro di assorbimento dei gas nobili presso il limite dello spettro d'arco | journal=Il Nuovo Cimento | publisher=Springer Science and Business Media LLC | volume=12 | issue=3 | year=1935 | issn=0029-6341 | doi=10.1007/bf02958288 | pages=154–161 | s2cid=119640917 | language=it}}</ref> in the sixties using synchrotron radiation at the National Bureau of Standards it was established that the broad asymmetric absorption peaks are due to [[Fano resonance]]s above the atomic ionization potential where the final states are many body quasi-bound states (i.e., a doubly excited atom) degenerate with the continuum.<ref name="fano3">{{cite journal | last1=FANO | first1=U. | last2=COOPER | first2=J. W. | title=Spectral Distribution of Atomic Oscillator Strengths | journal=Reviews of Modern Physics | publisher=American Physical Society (APS) | volume=40 | issue=3 | date=1968-07-01 | issn=0034-6861 | doi=10.1103/revmodphys.40.441 | pages=441–507}}</ref>