[Wien] Difference among XAS, ELNES and EELS

Thu Dec 6 11:11:55 CET 2007

I just want to add one point to this answer.
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)
If you consider that espilon1 is very close to 1, then one can 
reasonably assume the EELS is also proportional to epsilon2.
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.
I suggest that you look at the paper:
Phys. Rev. B *74*, 115106 (2006)
doi: <http://scitation.aip.org/jhtml/doi.jsp>10.1103/PhysRevB.74.115106

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.
The difference between both quantity are the "local field effect" and 
they can be very important in some cases.
See for instance :
Phys. Rev. Lett. 88, 037601 (2002)

Regards
Florent

Michel Jaouen a écrit :
>         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
>>>  
>>>        
>>>
>>> ------------------------------------------------------------------------
>>> ++¯ì¸âÎå'üý¦...â^îNª?ÅC"ÆéÊâtèª¦'é¶äÌÅI 
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>>
>>
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>
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>   
> __________________________________________
>
> Michel Jaouen
> Universite de Poitiers - UFR Sciences - SP2MI
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