Double-quantum-coherence attosecond x-ray spectroscopy of spatially   separated, spectrally overlapping core-electron transitions
  X-ray four-wave mixing signals generated in the $k_1 + k_2 - k_3$ phase-matching direction are simulated for N1s transitions in para-nitroanline and two-ring hydrocarbons disubstituted with an amine and a nitroso groups. The two-dimensional x-ray correlation spectra (2DXCS) provide background-free probes of couplings between core-electron transitions even for multiple core shells of the same type. Features attributed to couplings between spatially-separated core transitions connected by delocalized valence excitations provide information about molecular geometry and electronic structure unavailable from linear near-edge x-ray absorption (XANES). 
Igor V. Schweigert and Shaul Mukamel