[Wien] Difference among XAS, ELNES and EELS

[Michel Jaouen]

	Dear Bokang and Ashley,

>
>Here is my slight & probably incomplete answer 
>(I am not sure how much you already know):
>
>
>XAS and EELS give very similar experimental 
>spectra, and are based on similar physical 
>processes, but not exactly the same.
>
>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).

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.

>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.

Right. For x-ray, ELNES = XANES. TELNES2 is dedicated to EELS rather than XAS.

>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."

The difference is in the q dependance evoked above.

Best regards,

Michel

>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>
>On Dec 4, 2007, at 2:05 PM, îg çN wrote:
>
>>Dear wien users,
>>
>>         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?
>>
>>        Any reply will be appreciated!
>>
>>                                                BoKang
>>
>>
>>
>>
>><http://cn.mail.yahoo.com/promo/carnival07/index.html?source=xy>‡¯ì¸âÎå'üý¦
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-- 
__________________________________________

Michel Jaouen
Universite de Poitiers - UFR Sciences - SP2MI
LMP UMR 6630-CNRS
Boulevard Pierre et Marie Curie - Teleport 2
BP 30179
86962 Futuroscope - Chasseneuil Cedex
France
Tel : (33) 5 49 49 67 37
Fax : (33) 5 49 49 66 92
e-mail : Michel.Jaouen at univ-poitiers.fr
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