Onset and Near Threshold Evolution of the CuKαX-Ray Satellites

The evolution of the $\mathrm{Cu}K{\alpha }_{3,4}$ spectrum is studied from threshold to saturation using synchrotron radiation photoexcitation. A pure shake-off behavior is observed, with a smooth and monotonic intensity rise from zero to saturation, confirming theoretical predictions. Different onset energies are observed for the spectral features, but the shape of the spectrum is found to be invariant for energies higher than 50 eV above threshold. Full-spectrum fits based on ab initio Dirac-Fock calculations and measured onset energies conclusively show the satellites to result from $2p$ spectator transitions only, as predicted.