The electronic structure of the honeycomb lattice iridates Na2IrO3 and
Li2IrO3 has been investigated using resonant inelastic x-ray scattering (RIXS).
Crystal-field split d-d excitations are resolved in the high-resolution RIXS
spectra. In particular, the splitting due to non-cubic crystal fields, derived
from the splitting of j_eff=3/2 states, is much smaller than the typical
spin-orbit energy scale in iridates, validating the applicability of j_eff
physics in A2IrO3. We also find excitonic enhancement of the particle-hole
excitation gap around 0.4 eV, indicating that the nearest-neighbor Coulomb
interaction could be large. These findings suggest that both Na2IrO3 and
Li2IrO3 can be described as spin-orbit Mott insulators, similar to the square
lattice iridate Sr2IrO4
