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In Ca2RuO4, the competition of spin-orbit coupling $\zeta$ and tetragonalcrystal field splitting $\Delta_{CF}$ has been discussed controversially formany years. The orbital occupation depends on $\Delta_{CF}/\zeta$, which allowsus to address this ratio via the optical spectral weights of the lowestintersite Mott-Hubbard excitations. We study the optical conductivity ofCa$_2$Ru$_{0.99}$Ti$_{0.01}$O$_4$ in the range of 0.75 - 5 eV by ellipsometry,using the large single crystals that can be grown for small Ti concentrations.Based on a local multiplet calculation, our analysis results in $2.4 \leq\Delta_{CF}/\zeta \leq 4$ at 15 K. The dominant crystal field yields a groundstate close to xy orbital order but spin-orbit coupling is essential for aquantitative description of the properties. Furthermore, we observe apronounced decrease of $\Delta_{CF}$ with increasing temperature, as expectedbased on the reduction of octahedral distortions.

Spin-orbit coupling and crystal-field splitting in Ti-doped Ca2RuO4 studied by ellipsometry