  We report a comprehensive resonant inelastic x-ray scattering (RIXS) study of
various iridate materials focusing on core-level excitations and transitions
between crystal-field split d-levels. The 2p core hole created at the Ir $L_3$
absorption edge has a very short lifetime giving rise to a broad absorption
width ($\sim 5$~eV). This absorption linewidth broadening can be overcome by
studying the resonant x-ray emission spectroscopy (RXES) map, which is a
two-dimensional intensity map of the Ir L$\alpha_2$ emission obtained with high
energy-resolution monochromator and analyzer. By limiting the emitted photon
energy to a narrow range, one can obtain x-ray absorption spectra in the high
energy-resolution fluorescence detection (HERFD) mode, while one can also
simulate quasi-$M_4$-edge absorption spectra by integrating over incident
photon energies. Both methods improve the absorption line width significantly,
allowing detailed studies of unoccupied electronic structure in iridates and
other $5d$ transition metal compounds. On the other hand, the short lifetime of
the 2p core hole benefits the study of excitations of valence electrons. We
show that the incident energy dependence of the RIXS spectra for $d-d$
transitions is simple to understand due to the short core-hole lifetime, which
validates ultra-short core-hole lifetime approximation used widely in
theoretical calculations. We compared $d-d$ excitations in various iridates and
found that the excitations between the t$_{2g}$ and e$_g$ states share many
similarities among different materials. However, the RIXS spectra due to the
transitions between the spin-orbit-split t$_{2g}$ levels vary widely depending
on the oxidation state and electronic bandwidths.
