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Neutron scattering is used to investigate the single-ion spin and orbitalexcitations below the Mott-Hubbard gap in CoO. Three excitations are reportedat 0.870 $\pm$ 0.009 eV, 1.84 $\pm$ 0.03, and 2.30 $\pm$ 0.15 eV. These wereparameterized within a weak crystal field scheme with an intra-orbital exchangeof $J(dd)$=1.3 $\pm$ 0.2 eV and a crystal field splitting 10Dq=0.94 $\pm$ 0.10eV. A reduced spin-orbit coupling of \lambda=-0.016 $\pm$ 0.003 eV is derivedfrom dilute samples of Mg$_{0.97}$Co$_{0.03}$O, measured to removecomplications due to spin exchange and structural distortion parameters whichsplit the cubic phase degeneracy of the orbital excitations complicating theinelastic spectrum. The 1.84 eV, while reported using resonant x-ray andoptical techniques, was absent or weak for non resonant x-ray experiments andoverlaps with the expected position of a $^{4}A_{2}$ level. This transition isabsent in the dipolar approximation but expected to have a finite quadrupolarmatrix element that can be observed with neutron scattering techniques atlarger momentum transfers. Our results agree with a crystal field analysis (interms of Racah parameters and Tanabe-Sugano diagrams) and with previouscalculations performed using local-density band theory for Mott insulatingtransition metal oxides. The results also demonstrate the use of neutronscattering for measuring dipole forbidden transitions in transition metal oxidesystems.

Neutron scattering measurements of $dd$ and spin-orbit excitations below the Mott-Hubbard gap in CoO