Angle-resolved electron energy-loss spectroscopy investigation of crystal-field transitions on MnO and NiO surfaces: Exchange scattering versus direct scattering

In electron energy-loss spectroscopy, the excitation of low energy transitions, such as the crystal-field transitions in $3d$ transition-metal oxides, takes place via two scattering mechanism, namely, direct scattering and exchange scattering. The angular cross sections of both scattering mechanisms have been investigated by angular resolved electron energy-loss spectroscopy by comparing the angular intensity distributions of the crystal-field transitions on MnO and NiO. In MnO, the spin-forbidden sextet quartet are excited only by electron exchange scattering with nonuniform cross sections, resulting in nearly isotropic angular intensity distributions for extended energy-loss ranges. In NiO, the excitation of the spin-allowed triplet-triplet transitions is also possible by direct scattering for which the forward scattering is additionally modulated by the reciprocal surface lattice. The angular intensity distributions for extended energy-loss ranges in NiO display a large low-energy electron diffraction-like anisotropy from direct scattering on a broad exchange-scattering background.

Figure 1

Experimental setup as described in the text.

Figure 2

EELS spectra of MnO/Ag(001) and NiO/Ag(001).

Figure 3

(Color online) Elastic and inelastic-scattering distributions for MnO/Ag(001), $E_0=30\text{eV}$, and $\alpha =123°$. (a) Elastic intensity at $\Delta E=0$ with $\text{FWHM}\sim 230\text{meV}$. (b) Polar LEED distribution for the elastic electrons from (a) along (100). Red dashed lines represent the LEED directions. Spots are labeled by the reciprocal surface lattice. The surface normal is along the (001) axis. (c) Same as (b) along the (110) axis. (d) First group of CF transitions of MnO, ranging from 1.6 to 4.2 eV. (e) Polar intensity distributions of energy-loss range from (d) along (100). The blue dotted lines represent the angular transmission of the setup (see text). (f) Same as (e) along the (110) axis. (g) Second group of CF transitions of MnO, ranging from 4.2 to 5.6 eV. (h) Same as (e) for energy-loss range from (g). (i) Same as (f) for energy-loss range from (g).

Figure 4

(Color online) Elastic and inelastic-scattering distributions for NiO/Ag(001), $E_0=30\text{eV}$, and $\alpha =123°$. (a) Elastic intensity at $\Delta E=0$ with $\text{FWHM}\sim 230\text{meV}$. (b) Polar LEED distribution for the elastic electrons from (a) along (100). Red dashed lines represent the LEED directions. Spots are labeled by the reciprocal surface lattice. The surface normal is along the (001) axis. (c) Same as (b) along the (110) axis. (d) First group of CF transitions of NiO, ranging from 0.8 to 2.0 eV. (e) Polar intensity distributions of energy-loss range from (d) along (100). (f) Same as (e) along the (110) axis; (g) Second group of CF transitions of NiO, ranging from 2.5 to 4.2 eV. (h) Same as (e) for energy-loss range from (g). (i) Same as (f) for energy-loss range from (g).