A typical electron energy loss spectrum is shown below. It
consists of three parts:
peak at 0 eV:
It mainly contains electrons that still have the original beam
energy E0, i.e., they have only interacted
elastically or not at all with the specimen. In thin specimens,
the intensity of the zero-loss beam is high, so that damage
of the CCD chip can occur. Since the zero-loss beam contains not much useful information, it is often omitted during spectrum
region (< 100eV)
the electrons that have induced plasmon oscillations
occur. Since the plasmon generation is the most frequent inelastic
interaction of electron with the sample, the intensity in this
region is relatively high. Intensity and number of plasmon
peaks increases with specimen thickness.
region (> 100eV)
the ionization of atoms, a specific minimum energy, the critical
ionization energy EC or ionization threshold,
must be transferred from the incident electron to the expelled
inner-shell electron, which leads to ionization edges in the spectrum
at energy losses that are characteristic for an element. Thus,
X-ray spectroscopy, and it can be utilized
for qualitative and quantitative element analysis as well. In particular,
the detection of light elements is a main task of EELS.
to the plasmon generation, the inner-shell ionization is a much
less probable process, leading to a low intensity of the peaks.
In the high-loss region, the amount of inelastically scattered
electrons drastically decreases with increasing energy loss, thus
small peaks are superimposed on a strongly decreasing background
(s. spectrum). Because of the low intensity, the representation
of the high-loss region is often strongly enhanced (here: intensity
lot of valuable information is present in a electron energy loss spectrum.
energy resolution (below 1 eV) is much better than in X-ray
spectroscopy, and as a result more structural information can
be obtained from the fine structure in EELS. The
critical ionization energy EC is
sensitive to the chemical situation of the element: e.g., the
L edge of Cu metal and CuO are shifted in respect to each other
(chemical shift). Moreover, the
ionization process may take more energy than EC,
and therefore there also is intensity located after the
edge onset. Actually,
this region, designated as ELNES (Energy-Loss Near-Edge Structure),
mirrors the DOS and provides information about the bonding situation.
Modulations further away from the ionization edge contain information
about interatomic distances and coordination (EXELFS, Extended
Energy-Loss Fine Structure).
For many questions,
it is important to get this information
with a high local resolution. Such mappings
can be done with two different methods:
1. STEM with EELS
In STEM mode, an region of the sample is selected, and an EELS is measured
on each spot of the defined grid (serial measurement).
2. EFTEM using an energy filter
An energy range is selected, and an energy-filtered image is recorded
with electrons of this energy (parallel measurement).
EEL spectra are available in a database. Further information
can be found at Gatan's EELS page, the FELMI page of the TU Graz and at the USTEM page
of the TU Wien.