absorption spectrum. The modulation frequency corre-
sponds directly to the distance of the surrounding
atoms, while the amplitude is related to the type and
number of atoms. EXAFS studies are a probe of the
local structure. EXAFS can be applied to systems that
have local structure, but not necessarily long-range
structure, such as noncrystalline materials. In particu-
lar, bond lengths and local symmetry (COORDINATION
numbers) may be derived. The X-ray absorption spec-
trum may also show detailed structure below the
absorption edge. This X-ray absorption near edge
structure (XANES) arises from excitation of core elec-
trons to high-level vacant orbitals.
external return SeeION-PAIR RETURN.
extinct species A species no longer in existence.
extrusion transformation A TRANSFORMATION in
which an atom, or GROUPY, connected to two other
atoms, or groups X and Z, is lost from a molecule,
leading to a product in which X is bonded to Z, for
exampleThe reverse of an extrusion is called INSERTION.
See alsoCHELETROPIC REACTION.extrusion transformation 103