Determine the Chemical State Sample

Beyond simple composition analysis, the final properties of your material are determined by the chemical state and bonding of the atoms in the material. Learn how the strong coupling of the local density of states to the EELS signal enables you to determine the chemical state of your elements.

ELNES & EXELFS represented on spectrum.
ELNES & EXELFS represented on spectrum.

You can accurately deduce the chemical state of samples by analyzing the energy loss near edge structure (ELNES), which directly relates to the density of unoccupied states. The atom specific radial distribution function (RDF) is directly related to the extended energy loss fine structure (EXELFS).

ELNES reflects the local density of unoccupied states

ELNES reflects the site-specific density of states

The initial core state is localized to a particular atom, thus greatest overlap occurs with unoccupied outer-shell orbitals at that atom.

ELNES reflects the symmetry-projected density of states

Under the dipole approximation, the initial and final states must differ in their quantum state number (l) by ±1. For example, ELNES of a K-­edge (initial s-state) will couple to p-­like final states, while ELNES of an L-­edge (initial p-state) will couple to d­-like or s-like final states: ELNES of a K-edge (initial s-state) mainly features p-like final states, while ELNES of an L-edge (initial p-state) will mainly feature d-like final states.

ELNES can be orientation dependent

In anisotropic crystalline materials, ELNES changes with the alignment of the momentum transfer along different crystal axes.

Chemical shifts

Redistribution of valence charge will alter core level screening, and thus change the potential energy of initial core states. The final energy states will change depending on the band structure of the material (e.g., Fermi level shift or opening of a band gap in the material). Both of these effects will cause the edge thresholds to shift.

ELNES and chemical shifts
"Accurate chemical shift measurements using a post-column spectrometer equipped with an experimental electrostatic shutter." G. Kothleitner et al., EDGE Banff 2009.

Atomic specific radial distribution function (RDF)

Atomic specific radial distribution function
The extended fine structure represents transitions from the core level to states above the vacuum level. This creates a source of free electrons in the material centered at the interacting atom. These electrons scatter and interact giving rise to interference terms in the tail of the scattering.