We present a theory of the resonant inelastic x-ray scattering (RIXS) to
study electronic excitations in orbital ordered manganites. The charge and
orbital excitations of the Mn 3d electron are caused by the Coulomb
interactions in the intermediate scattering state. The scattering cross section
is formulated by the Liouville operator method where the local and itinerant
natures of the excitations are taken into account on an equal footing. As a
result, the cross section is expressed by the charge and orbital correlation
functions associated with local corrections. The RIXS spectra are calculated
numerically as functions of momentum and polarization of x ray. Through the
calculations, we propose that RIXS provides a great opportunity to study the
unique electronic excitations in correlated electron systems with orbital
degeneracy.Comment: 8 pages, 5 figure