  An application of resonant inelastic x-ray scattering technique for studying
of optical scale excitations in electron-correlated materials is discussed.
Examples are given including data obtained for 3d transition metal, lanthanide,
and actinide systems. In some cases, the data are compared with the results of
crystal-field multiplet and Anderson impurity model calculations. Advantages of
this technigue are pointed out, such as an ability to probe an extended
multiplet structure of the ground state configuration, which is not fully
accessible by other spectroscopies, an extreme sensitivity of spectral profiles
to the chemical state of the element in question and to the crystal-field
strength, and a great potential in probing the ground state character (for
example, ground state J-mixing in rare-earths) due to the technique's elemental
selectivity and strict selection rules. Issues are addressed, such as a
possible deviations from the linear dispersion of inelastic scattering
structures, corresponding to charge-transfer excitations, with varying
excitation energies and an estimation of values for model parameters, involved
in the description of charge-transfer processes
