Why are core electrons removed from atoms on X-ray irradiation in XPS?

[madhusoodan]

[Mentor's note: Post moved to its own thread]

Hi
Please let me know why core electrons are removed from the atoms on x-ray irradiation in XPS, though it was easier to outer electrons?

 

[jedishrfu]

Not knowing the answer to your question, I found this SLAC writeup that may help:

http://ssrl.slac.stanford.edu/nilssongroup/corelevel.html

and this wikipedia article:

https://en.wikipedia.org/wiki/Absorption_spectroscopy

 

[DrDu]

Weakly bound electrons can be treated like free electrons, as the binding energy is much smaller than the X-ray energy.
For free electrons, scattering from x-rays is not possible as this would violate momentum conservation.
So scattering or excitation of electrons is only possible if they are appreciably bound to the nucleus.

 

[ZapperZ]

[Mentor's note: Post moved to its own thread]

Hi
Please let me know why core electrons are removed from the atoms on x-ray irradiation in XPS, though it was easier to outer electrons?

I think there is a misunderstanding here.

The core level electrons is used to distinguished the electrons that are highly localized to the parent atoms or ions of the solid, as opposed to the "band" electrons that are due to the overlapping of the outer orbitals in the formation of the solid. These band electrons usually show a dispersive feature that are not observed in the core-level electrons. So these are not simply "outer electrons" as in outer shell. There are no isolated "outer shell" electrons anymore in a solid.

XPS will excite all electron levels that are possible. However, the cross-section for absorption is different for different levels and different photon energies. There is also a difference in the density of states, or the density of levels. All of these will dictate the signal or peak strength for each level. You CAN see the Fermi edge at low binding energy. This means that you do get photoemission from the band electrons ( your outer electrons), but the signal is often weak when compared with the core level electrons, which usually dominate.

Zz.

 

[madhusoodan]

Thanks for clearing my earlier doubt.

Why can't Cu K alpha eject electrons from atoms of the sample in XRD?
As everyone knows, Al K alpha photon with 1486 eV energy is used for XPS to remove electrons from core levels. My doubt is why can't Cu K alpha photon with 8000+ eV do that?

 

[Vagn]

Thanks for clearing my earlier doubt.

Why can't Cu K alpha eject electrons from atoms of the sample in XRD?
As everyone knows, Al K alpha photon with 1486 eV energy is used for XPS to remove electrons from core levels. My doubt is why can't Cu K alpha photon with 8000+ eV do that?

Cu Kα can be used for XPS, as mentioned in this paper. However, even monochromated Cu Kα suffers from having a fairly large line width at ≈2.4 eV, in comparison monochromated Al Kα has a resolution <1 eV. XRD experiments using a lab source would usually be done in ambient conditions, so you wouldn't detect the emitted photoelectrons.
Typically for high energy x-rays, people go to a synchrotron light source (and in the future, probably an x-ray free electron laser source) instead.

 

[madhusoodan]

Cu Kα can be used for XPS, as mentioned in this paper. However, even monochromated Cu Kα suffers from having a fairly large line width at ≈2.4 eV, in comparison monochromated Al Kα has a resolution <1 eV. XRD experiments using a lab source would usually be done in ambient conditions, so you wouldn't detect the emitted photoelectrons.
Typically for high energy x-rays, people go to a synchrotron light source (and in the future, probably an x-ray free electron laser source) instead.

Thanks for the reply. Still I doubt whether electrons are ejected out from the atoms of sample or not during the xrd. Since energy of the Cu Kα photons is huge compared to Al kα photon,it seems electrons seems to be get ejected, however it is not observed and mentioned anywhere. Please help me to get clear picture of their differences.