Evolution from threshold of a hollow atom's X-ray emission spectrum: The CuKh alpha(1,2) hypersatellites

The first pure high-resolution photoexcited K(h)alpha(1,2) hypersatellite s pectrum from a hollow Cu atom is measured. Its K(h)alpha(2)-K alpha(1) shif t and the K(h)alpha(1,2) lines' splitting, widths, and intensity ratio are accurately determined. Such spectra are uniquely suited to study reIativist ic correlation effects, transition from LS to intermediate coupling, and th e Breit-Wigner interaction. The threshold energy and the spectrum's evoluti on with excitation energy are also measured. The roles of relativity and Br eit interaction are explored by comparison with ab initio Dirac-Fock calcul ations.