Evolution from threshold of a hollow atom's X-ray emission spectrum:: The CuKhα1,2 hypersatellites

The first pure high-resolution photoexcited K(h)alpha(1,2) hypersatellite spectrum from a hollow Cu atom is measured. Its K(h)alpha(2)-K alpha(1) shift and the K(h)alpha(1,2) lines' splitting, widths, and intensity ratio are accurately determined. Such spectra are uniquely suited to study reIativistic 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.