[Wien] Difference among XAS, ELNES and EELS

[Laurence Marks]

To chirp in with two small, but important points:
1) 99% of EELS nowadays is done with electron beams < 1nm in diameter,
so is spatially resolved.
2) In EELS one MUST consider dynamical diffraction of the electrons,
which is currently not included in TELNES and most codes.

On Dec 4, 2007 9:55 AM, Michel Jaouen <Michel.Jaouen at univ-poitiers.fr> wrote:
>
>
>         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
>
>
>
>
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>  --
>
>
> __________________________________________
>
>
> Michel Jaouen
> Universite de Poitiers - UFR Sciences - SP2MI
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> e-mail : Michel.Jaouen at univ-poitiers.fr
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-- 
Laurence Marks
Department of Materials Science and Engineering
MSE Rm 2036 Cook Hall
2220 N Campus Drive
Northwestern University
Evanston, IL 60208, USA
Tel: (847) 491-3996 Fax: (847) 491-7820
email: L-marks at northwestern dot edu
Web: www.numis.northwestern.edu
Commission on Electron Diffraction of IUCR
www.numis.northwestern.edu/IUCR_CED