Unresolvable Rydberg lines in X-ray absorption spectra of free atoms

The authors investigate the contribution of the unseparated Rydberg states which are usually neglected, in atomic K-edge X-ray spectra. They suppose that the core hole broadening is large, so that the global contribution of all unresolved Rydberg levels introduces an apparent threshold shift with a value nearly equal to the separation of the first unresolvable Rydberg line from the series limit. The apparent threshold has the same dependence on the photon energy as the true one. The region of unresolvable Rydberg lines can be described by means of a quasi-particle approach, by introducing a pseudo-wave vector formalism. The authors perform an analysis of Ne and Ar K-edge absorption spectra. Using the present theory, they derive a very simple procedure for the evaluation of the instrumental and the core hole broadening in X-ray spectra.