X-ray scattering spectra of Cu and Ni metals have been measured using monochromatic synchrotron radiation tuned from far above to more than 10 eV below threshold. Energy conservation in the scattering process is found to be sufficient to explain the modulation of the spectral shape, neglecting momentum conservation and channel interference. At excitation energies close to and above threshold, the emission spectra map the occupied local partial density of states. For the subthreshold excitations, the high-energy flank of the inelastic scattering exhibits a Raman-type linear dispersion, and an asymmetric low-energy tail develops. For excitation far below threshold the emission spectra are proportional to a convolution of the occupied and unoccupied local partial densities of states.
Magnuson, M., Rubensson, J. E., Föhlisch, A., Wassdahl, N., Nilsson, A., & Mårtensson, N. (2003). X-ray fluorescence spectra of metals excited below threshold. Physical Review B - Condensed Matter and Materials Physics, 68(4). https://doi.org/10.1103/PhysRevB.68.045119