[Wien] Calculating XPS spectra in Wien2k

[Laurence Marks]

We need to differentiate two things:
a) How one removes any offsets experimentally with an insulator. Often
cleaning the sample is not an option -- this may also destroy it.
b) What reference level one should use in a Wien calculation: the T=0
Fermi energy, the T=Room Temperature Fermi Energy (TEMP/TEMPS), a core
level well away from the target atom (for a supercell), the vacuum
level (if available)?

One has to be careful of possible artifacts due to a change in E_F in
the cell because there is not a large enough bulk reservoire to set it
correctly, artifacts in the total energy due to charged cell issues
(which I think the Slater method avoids), artifacts due to
interactions between core-holes (or half holes) in adjacent
supercells, plus probably others I've not thought of yet.

On Thu, Sep 18, 2008 at 8:17 AM, Pierre-Emmanuel Lippens
<lippens at univ-montp2.fr> wrote:
> At 11:19 18/09/2008, you wrote:
>>Realy not a good idea,
>>  if you have C on the surface, what C is it ? CO, hydrocarbon,
>> graphite ? All have a different binding energy of the C 1s !
>>I suggest, if youre sample is not to be supposed to contain C, you
>>should clean it.
>
>
> This is not my own choice (I am not XPS experimentalist) but what I
> have read in some XPS papers.
>
> The main point: experimental calibration of the binding energy scale
> requires a reference energy.
>
> C1s is surely not the most accurate choice. From discussions with XPS
> users: this peak is used because, if you don't clean your C free
> sample, you have C that may originate from contamination in the
> analysis chamber (vacuum pump oil) and this C is not expected to form
> chemical bonds with the sample.
>
> But as you said in your email you can have other C on the surface and
> the reference is not expected to be very accurate.
>
>
>>XPS energies have to be calibrated using the Au (or Ag) Fermi energy !!!!!
>
> A better choice than C1s but unfortunately not used by all the XPS
> experimentalists...
>
>
>
>>and the other question: And for an insulator when the extra 1/2 (or
>>1) electron goes into the
>>conduction band and E_F changes....?
>>An insulater will most probably charge if you do XPS, and the
>>energies do not tell you much.
>
> Can we consider that charge mainly shifts the energy scale? What is
> the effect on the binding energy differences for a given sample?
>
> Pierre
>
>
>
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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
Chair, Commission on Electron Crystallography of IUCR
www.numis.northwestern.edu/IUCR_CED