  Low-energy electron excitation spectra were measured on a single crystal of a
typical iron-based superconductor PrFeAsO$_{0.7}$ using resonant inelastic
X-ray scattering (RIXS) at the Fe-$L_3$ edge. Characteristic RIXS features are
clearly observed around 0.5, 1-1.5 and 2-3 eV energy losses. These excitations
are analyzed microscopically with theoretical calculations using a 22-orbital
model derived from first-principles electronic structure calculation. Based on
the agreement with the experiment, the RIXS features are assigned to Fe-$d$
orbital excitations which, at low energies, are accompanied by spin flipping
and dominated by Fe $d_{yz}$ and $d_{xz}$ orbital characters. Furthermore, our
calculations suggest dispersive momentum dependence of the RIXS excitations
below 0.5 eV, and predict remarkable splitting and merging of the lower-energy
excitations in momentum space. Those excitations, which were not observed in
the present experiment, highlight the potential of RIXS with an improved energy
resolution to unravel new details of the electronic structure of the iron-based
superconductors.
