  We examine the role of spin-orbit coupling in the electronic structure of
$\alpha$-RuCl3, in which Ru ions in 4d5 configuration form a honeycomb lattice.
The measured optical spectra exhibit a clear optical gap and excitations within
the t2g orbitals. The spectra can be described very well with first-principles
electronic structure calculations obtained by taking into account both spin
orbit coupling and electron correlations. Furthermore, our X-ray absorption
spectroscopy measurements at the Ru L-edges exhibit distinct spectral features
associated with the presence of substantial spin- orbit coupling, as well as an
anomalously large branching ratio. We propose that $\alpha$-RuCl3 is a
spin-orbit assisted Mott insulator, and the bond-dependent Kitaev interaction
may be relevant for this compound.
