Chapter/Index: Introduction | A |
B |
C |
D |
E |
F |
G |
H |
I |
J |
K |
L |
M |
N |
O |
P |
Q |
R |
S |
T |
U |
V |
W |
X |
Y |
Z |
Appendix
Spread of Kinetic Electron Energy
| Energy spread (ΔE) of the electron beam is a measure of the temporal coherency. This spread is important in EELS. The only way to measure the energy spread is to use an electron spectrometer by taking advantage of high energy resolution of EELS (~ 0.1 eV). Therefore, the value of ΔE can be simply measured by collecting a spectrum of electrons without a specimen in the way of the beam. The spectrum then consists of a single Gaussian peak and the resolution of the spectrum is defined as the FWHM of this peak. Typical values of ΔE for the various electron sources are 3 eV for tungsten (W) gun, 1.5 eV for LaB6 gun, 0.7 eV for Schottky FEG, and 0.3 eV for cold FEG at accelerating voltage of 100 kV.
In order to improve the energy spread of the source, monochromators have
recently been introduced. The energy spread with a monochromator for any source can be reduced to < ~100 meV. There are two basic approaches: the Wien filter with
crossed electric and magnetic fields and the electrostatic Omega filter. Both types
improve the energy width to about 0.2 eV at a yet acceptable beam current of
several 100 pA. The singleWien filter limits the probe size to about 2 nm, whereas
there is no such limitation in the case of the Omega filter. This spatial aberration
of the Wien filter can be improved by using double Wien filters [1].
[1] Martínez G, Tsuno K. 2004. Design of
Wien filters with high resolution. Ultramicroscopy
100:105–14
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