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I just want to add one point to this answer.<br>
When you do XAS measurments, the double differential cross section is
proportional to epsilon2 while for the EELS, the double differential
cross section is formally proportional to Im(-1/epsilon)<br>
If you consider that espilon1 is very close to 1, then one can
reasonably assume the EELS is also proportional to epsilon2.<br>
However, if you look for low energy losses (between 1 to 100eV), the
epsilon1 is no more equal to 1 and polarization effects can be very
important.<br>
I suggest that you look at the paper:<br>
Phys. Rev. B <b>74</b>, 115106 (2006)<br>
<a href="http://scitation.aip.org/jhtml/doi.jsp" title="doi">doi:</a>10.1103/PhysRevB.74.115106<br>
<br>
Don't forget also that when we do the calculation of epsilon, we
calculate a microscopic quantity but we are in fact interested in
simulating a macroscopic one.<br>
The difference between both quantity are the "local field effect" and
they can be very important in some cases.<br>
See for instance :<br>
Phys. Rev. Lett. 88, 037601 (2002)<br>
<br>
Regards<br>
Florent<br>
<br>
<br>
<br>
Michel Jaouen a écrit :
<blockquote type="cite">
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<title>Re: [Wien] Difference among XAS, ELNES and
EELS</title>
<div><x-tab> </x-tab>Dear
Bokang and Ashley,</div>
<div><br>
</div>
<blockquote type="cite" cite=""><br>
</blockquote>
<blockquote type="cite" cite="">Here is my slight & probably
incomplete answer (I am not sure how much you already know):<br>
</blockquote>
<blockquote type="cite" cite=""><br>
</blockquote>
<blockquote type="cite" cite="">XAS and EELS give very similar
experimental spectra, and are based on similar physical processes, but
not exactly the same.<br>
</blockquote>
<blockquote type="cite" cite="">In XAS, a photon is absorbed by the
sample and, in transmission experiments, the resulting attenuation of
the beam is measured while the photon energy is scanned (~ 0.2 - 40
keV depending on the beamline). In EELS in a TEM, the electron
beam is held at one energy (e.g. ~ 300 kV) and as electrons pass
through the sample, energy is lost. That energy loss
is measured in Electron Energy Loss Spectroscopy (detection of ~
0 - 2000 eV).</blockquote>
<div><br>
</div>
<div>As noted by Ashley, most people consider that XAS and EELS are
basically the same in the sense they carry the same information. But
from principle they are not. In XAS the probe (photon) is absorbed
(thus destroyed) whereas in EESL the electron is ineslatically
scattered. What is really similear to EELS with photons is non
resonant x-ray inelestic scattering (NRIXS ou Raman x-ray scattering)
where the incident photon energy is fixed (elastic: Rayleigh, plasmon:
Compton, etc). From a theoretical point of view both EELS and NRIXS
crosss-sections are expressed as the product of a prefactor (e-e
Rutherford cross-section for EELS, Thomson cross-section for NRIXS
with the dynamic structure factor S(q, E) which is the quantity of
interest. In the dipole limit (q->0) these inelastic loss
spectroscopies are proportional to XAS. EELS and NRIXS yield
additional information for finite momentum transfer (monopole,
quadrupole ans higher couplings). But due to collection apertures in
the TEM, the momentum transfer is small and it is the reason making
XAS and EELS nearly identical. On nowdays SR are developped beam lines
allowing to record simultaneously spectra for several momentum (and
quite large, much more than with EELS) transfert. If more interested
look at PRB 72, 045136 (2005) and refernces therein or ask for NRIXS
in your web browser.</div>
<div><br>
</div>
<blockquote type="cite" cite="">ELNES is a sub-set of EELS - the
Energy
Loss Near-Edge Structure, typically the part of the EEL spectrum <
50 eV beyond the edge onset. The setup of the TEM can affect the
EELS results, which is why there are adjustable parameters in the
w2web TELNES window. </blockquote>
<div><br>
</div>
<div>Right. For x-ray, ELNES = XANES. TELNES2 is dedicated to EELS
rather than XAS.</div>
<div><br>
</div>
<blockquote type="cite" cite="">My impression is that in XAS the
allowed
electronic transitions are more strictly governed by the dipole
selection rules, whereas in EELS, the rules can be bent a bit more
depending on the experiment. However, I have not found a lot in
literature that contrasts these two techniques - a lot of people just
acknowledge that they are "basically the same."</blockquote>
<div><br>
</div>
<div>The difference is in the q dependance evoked above.<br>
</div>
<div>Best regards,</div>
<div><br>
</div>
<div>Michel</div>
<div><br>
</div>
<blockquote type="cite" cite=""><font face="Arial" size="-1">~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~</font><br>
</blockquote>
<blockquote type="cite" cite=""><br>
</blockquote>
<blockquote type="cite" cite="">On Dec 4, 2007, at 2:05 PM, îg çN
wrote:</blockquote>
<blockquote type="cite" cite=""><br>
<blockquote type="cite" cite="">Dear wien users,</blockquote>
<blockquote type="cite" cite=""> </blockquote>
<blockquote type="cite" cite="">
Using WIEN2k code, we can calculate XAS and ELNES. I am very
confused, what's the differences among XAS, ELNES and EELS. Could
anyone tell me?</blockquote>
<blockquote type="cite" cite=""> </blockquote>
<blockquote type="cite" cite=""> Any
reply will be appreciated!</blockquote>
<blockquote type="cite" cite=""> </blockquote>
<blockquote type="cite" cite=""> <span></span> <span></span> <span></span> <span></span>
BoKang</blockquote>
<blockquote type="cite" cite=""> </blockquote>
<blockquote type="cite" cite="">
</blockquote>
<blockquote type="cite" cite=""><br>
</blockquote>
<blockquote type="cite" cite="">
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<pre>--
</pre>
<div align="center"><font color="#000000" face="Times">__________________________________________</font></div>
<div align="center"><font color="#000000" face="Times"><br>
</font></div>
<div align="center"><font color="#000000" face="Times">Michel
Jaouen</font></div>
<div align="center"><font color="#000000" face="Times">Universite de
Poitiers - UFR Sciences - SP2MI</font></div>
<div align="center"><font color="#000000" face="Times">LMP UMR
6630-CNRS</font></div>
<div align="center"><font color="#000000" face="Times">Boulevard
Pierre et Marie Curie - Teleport 2</font></div>
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30179</font></div>
<div align="center"><font color="#000000" face="Times">86962
Futuroscope - Chasseneuil Cedex</font></div>
<div align="center"><font color="#000000" face="Times">France</font></div>
<div align="center"><font color="#000000" face="Times">Tel : (33) 5
49
49 67 37</font></div>
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49
49 66 92</font></div>
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<a class="moz-txt-link-abbreviated" href="mailto:Michel.Jaouen@univ-poitiers.fr">Michel.Jaouen@univ-poitiers.fr</a></font></div>
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