Evolution from threshold of a hollow atom’s X-ray emission spectrum: The Cu K^hα_1,2hypersatellites

The first pure high-resolution photoexcited K^hα_1,2hypersatellite spectrum from a hollow Cu atom is measured. Its K^hα₂- K α₁shift and the K^hα_1,2lines’ splitting, widths, and intensity ratio are accurately determined. Such spectra are uniquely suited to study relativistic correlation effects, transition from LS to intermediate coupling, and the Breit-Wigner interaction. The threshold energy and the spectrum’s evolution with excitation energy are also measured. The roles of relativity and Breit interaction are explored by comparison with ab initio Dirac-Fock calculations.