  Ru M$_3$-edge resonant inelastic x-ray scattering (RIXS) measurements of
RuCl$_3$ with 27 meV resolution reveals a spin-orbit exciton without noticeable
splitting. We extract values for the spin-orbit coupling constant
($\lambda=154\pm2$ meV) and trigonal distortion field energy
($\left|\Delta\right|<65$ meV) which support the $j_{\rm eff}=1/2$ nature of
RuCl$_3$. We demonstrate the feasibility of M-edge RIXS for $4d$ systems, which
allows ultra high-resolution RIXS of $4d$ systems until instrumentation for
L-edge RIXS improves.
