Observation of $\mathit{dd}$ excitations in NiO and ${\mathrm{NiCl}}_2$ using $K$-edge resonant inelastic x-ray scattering

The presence of $\mathit{dd}$ excitations in $K$-edge resonant inelastic x-ray scattering in the $1s\to 4p$ region of transition-metal compounds and their excitation mechanism is established through measurements of NiO and NiCl${}_2$. It is demonstrated that the valence excitations are due to the interaction between the excited $4p$ electron and the $3d$ valence electrons. A detailed analytical framework for interpreting these excitations in transition-metal compounds is presented, demonstrating a strong angular dependence for different $\mathit{dd}$ excitations.

Figure 1

Ni $K$-edge RIXS of NiO[001] and NiCl${}_2$. The x-ray absorption is shown in the right panel. RIXS spectra for different excitation energy are shown in the left panel.

Figure 2

(a) The total RIXS intensity due to the $p$-$d$ Coulomb interaction between the excited $4p$ electron and the $3d$ valence electrons in the intermediate state. A ${90}^{°}$ horizontal scattering condition with the incoming polarization vectors ${45}^{°}$ with respect to the $z$ axis is used; see inset. (b) The RIXS intensity, but now separated into different terms in the $p$-$d$ Coulomb interaction, where $F^2$ is related to the direct interaction and $G^1$ and $G^3$ are exchange interactions. The top shows the angular distributions as a function of the direction of the incoming polarization vector.