RESONANT INELASTIC X-RAY-SCATTERING WITH SYMMETRY-SELECTIVE EXCITATION

We present a theoretical formulation of resonant inelastic x-ray scatt ering (RIXS) of molecules with an element of symmetry, and study the c onsequences of this formulation both with respect to short and long li fetimes of the intermediate core-excited scattering states and with re spect to localized versus delocalized descriptions for these states. S trong polarization and excitation energy dependencies are predicted. T he ''core-hole localization'' problem is explored also from the point of view of diffractional scattering of an x-ray photon showing that x- ray photons distinguish core holes in the short-wavelength limit only. By analyzing the properties of the x-ray scattering tensor, strict se lection rules for the RIXS process are derived. The local decompositio n model (one-center approximation) frequently employed in ordinary x-r ay emission is generalized for the calculation of intensities and pola rization dependence of the RIXS process, assuming either localized or delocalized core holes. It is argued that Stokes doubling should be a commonly observable feature in RIXS spectra. By means of model calcula tions we demonstrate the crucial role of RIXS selection rules, of chan nel interferences, of Stokes doubling, and of the form of the excitati on frequency function for the appearance of RIXS spectra.