1. XPS (X ray photoemission spectroscopy) /ESCA (Electron Spectroscopy for Chemical Analysis)

2. XPS
Core-levels: electronic
core-levels are more atomic-like: element
chemical shifts from formal oxidation
state of the atom, the local chemical and
physical environment : chemical
like Auger electron it has also short l: surface sensitive
Angular dependence has
diffraction effects: structure
Magnetic dichroism magnetic

3. Theoretical consideration
As photoemission is much more simple process than Auger process, conservation of energy then requires that :
KE = hn - ( E(A+ ) - E(A) ) - F
The final term in brackets,
representing the difference in energy between the ionized and neutral atoms, is generally called the binding energy (BE) of the electron . (F is the work function of the solid when KE is counted near surface, however, KE detected by analyzer then F is the work function of analyzer.)

4. Z dependence Elemental specific!
BE follows the energy levels: BE(1s)>BE(2s)>BE(2p)…
BE with same orbital increase with Z: BE(Mg1s)>BE(Na1s)
Elemental specific!
XPS data base

5. Main features of XPS

6. Three Step Model Absorption of the photon and excitation of electrons
2.Transport of electrons to the surface
3.The escape of the electrons from surface to the vacuum.

7. Inelastic scattering
In the step 2, inelastic scattering let XPS spectra consists of core-level photo-emission peaks imposed by a step-like structure (background) due to the various mechanism to lose kinetic energy. Besides, there are also AES processes visible.
XPS peak fit

8. XPS peak identification
Photoelectron lines: core-level, valence bands, spin-orbit splitting
Auger lines
Chemical shifts
X-ray satellites
X-ray “Ghost”
Shake-up satellite
Multiplet satellite
Energy loss lines

9. Spin-Orbit splitting:
Spin-orbit splitting is an initial state effect. For any electron in orbital with orbital angular momentum, coupling between magnetic fields of spin (s) and angular momentum (l) occurs
Lower binding energy
Higher binding energy

10. Total angular momentum j = |l ± s|, therefore for s electron there is no degeneracy, and other orbitals have two degeneracy:
- s orbitals are not spin-orbit split - singlet in XPS
- p, d, f… orbitals are spin-orbit split - doublets in XPS
- BE of lower j value in doublet is higher (BE 2p1/2 > BE 2p3/2)
- Magnitude of spin-orbit splitting increases with Z
- Magnitude of spin-orbit splitting decreases with distance from nucleus
(increased nuclear shielding)
Intensity ratio?

11. Core Level Chemical Shifts
Position of orbitals in atom is sensitive to chemical environment of atom. In solid all core levels for that atom shifted by approx. same amount (<10 eV). Chemical shift correlated with overall charge on atom (Reduced charge ®increased BE)
For k-shell of an atom in a compound:
EB(k) = EB(k,qA) +V
where EB(k,qA) is the binding energy of a free ion, A, qA is the net charge of A, and V is the potential at A due to all other atoms. V can be described as:
V = e2qASqi/riA
(i is any other atoms except A), riA is the distance between i and A, therefore chemical shift is:
DEB(k)=EB(k,qA1)-EB(k,qA2 )+V1 –V2
qA1 and qA2 is the difference of charge in two states.

12. XPS spectra for Si and its compounds with F in a) and chemical shifts vs. the charge in b)

13. Both S and Si binding energies increase with psitive charge (the loss of negative charge of electron), and the same for C.

14. ESCA (Electron Spectroscopy for Chemical Analysis)
The chemical shifts due to the variation of the distribution of the charges at the atom site is the main reason for the other name of XPS:
ESCA (Electron Spectroscopy for Chemical Analysis)
As the samples shown before, binding energies of Al3+ is higher than the metal atom, in the meanwhile, the binding energy of O atom (more positive charge) is higher than the O2- ion.

15. Shake-up and shake-off
Photoemission process can leave the ions in the ground state (main peak) and also possibly in an excited sate (shake-up/shake-off satellites), the latter makes the KE of photoelectron less: higher BE.
- excitation of electron to bound state shake-up satellite
- excitation of electron to unbound (continuum) state shake-off satellite
- excitation of hole state shake-down satellite - rare

16. The shown is XPS
spectra for Cu 2p
photoemission at
different chemical
states. The shake-up
Lines does not exist
in Cu metal, and
is unique for CuO
And CuSO4

17. Some general rules
Shake-up features especially common in transition metal oxides associated with paramagnetic species. Generally, the shake-up/shake-off satellites have intensities and energy separations from the parent photoelectron line that are unique to each chemical state, which can be used to analyze the chemical state of the elements. Even Some Auger lines also exhibit changes due to these processes. With transition metal, the absence of these lines is the fingerprint for elemental or diamagnetic states. Prominent satellites occurs with paramagnetic states.

18. Multiplet splitting and shake-up/shake-off lines are generally expected in the paramagnetic states:

19. MnO XPS spectra
Chemical shifts are too small to distinguish the chemical states of Mn in MnO from a). In b) the satellites are due to Mn2+, while for Mn3+ and Mn4+, although there should be satellites, they are with higher binding energies.
Shake-up/Shake-off satellites are another reason for the chemical sensitivity of XPS

20. Multiplet satellite
Following photoelectron emission, the remaining unpaired electron may couple with other unpaired electrons in the atom, resulting in an ion with several possible final state configurations with as many different energies. This produces a line which is split asymmetrically into several components.
For s-type orbital with other unpaired electrons in the atom there are split lines like in the shown Figure for Mn 3s.
For p or even higher orbital levels, is more complex and subtle

21. eph + esolid e*ph + e**solid Energy loss lines
Photoelectrons travelling through the solid can interact with other electrons in the material. These interactions can result in the photoelectron exciting an electronic transition, thus losing some of its energy (inelastic scattering). Most common are due to interband or plasmons (bulk or surface).
(bulk plasmon)
Surface plasmon

22. The plasmon loss satellites are rarely sharp in insulators but very prominent in the metals. The main peak is normally observed at higher binding energy with several lines with the same energy intervals and reduced intensity, and the interval can be not only single one due to different origins: bulk or surface plasmons, bulk one is more prominent and interval larger (21/2 factor of the surface one).

23. Energy of Light Wavelength () 106m 103m 1 m 10-3m 10-6m Energy
10-6eV
10-3eV
1 eV
1 KeV
1 MeV
Short wave radio
Broad-cast
Infrared
Gamma Ray
X-ray UV
Visible

24. X-ray tube Early x-ray source
Standard lab X-ray source is by very high energy e beam hitting the anode.

25. A common Dual anode X-ray tube

26. X-ray spectrum from x-ray tube
Characteristic lines from the X ray fluorescence process (XRF) and a broad background (Bremsstrahlung), which is strongly depends on the energy of the electron

27. Typical X-ray anode material (Mg and Al)
2p3/2 ® 1s and 2p1/2 ® 1s transitions produce soft x-rays
Ka1,2 radiation (unresolved doublet)
hn (eV) FWHM (eV) Mg Al
Same transitions in doubly ionized Mg or Al produce Ka3,4 lines at hn ~ 9-10 eV higher…
3p ® 1s transitions produce Kb x-rays
Energies and widths of characteristic soft X-ray lines of different materials

28. Mg K-shell X-ray emission spectrum.
The full line shows the characteristic line emissions after subtraction of a constant background as shown by the dashed line. Note the logarithmic intensity scale.

29. X-ray satellites
Emission from non-monochromatic x-ray sources produces satellite peaks in XPS spectrum at lower BE.

30. “ghost peaks”
O Ka at eV
Ghost peaks are due to contamination of the x-ray source, which produces x-ray emission at different wavelength and it can also due to contamination of the sample holder etc.

31. Monochromatic X-ray Goal to achieve Energy Analyzer Rowland Circle
Narrow peak width
Reduced background
No satellite & Ghost peaks
Goal to achieve
Energy
Analyzer
Rowland Circle
Quartz
Crystal Disperser e-
nl=2dsinq
For quartz (1010) surface, d=0.42 nm and 78.5 degree for Al Ka 0.93 nm
Sample
X-ray Anode

32. Synchrotron Radiation
The synchrotron storage ring is a tubular vacuum chamber made to:
Hold an electron beam travelling through it at nearly the speed of light. Maintain the high energy of the electron beam. As the accelerating electrons circle the ring at relativistic velocities, they give off intense beams of light including x-rays. By using a monochromator the light will be Monochromatic.
Key properties of synchrotron radiation:
high intensity
tunability in wide range
near-coherence
polarized.
pulsed
well collimated
NUS has such a source in Singapore!

33. Sample charging effects
The light for XPS always charges surface positively (shifting of spectrum to higher binding energy) and leads to general instability (spectral noise). For the metal sample, which can be grounded and the charges can be quickly gone. However, for insulator, this effects are serious and need to be treated.
For XPS (even AES) never forget ground the sample !!!
C 1s shifts due to the charging

34. Inhomogeneous Surface Charging
Charging can even change the line shape due to Inhomogeneous Surface Charging, which have different positive voltage on the surface.
In a lot of cases, there is only spectral shift due to charging, which can be determined by comparison with known elemental XPS lines, for example C 1s.

35. Charge Compensation
When the spectra is distorted
Other methods including make the sample very thin that is does not insulate, earthed metal mesh and very focused X-ray spot can also help sometimes.
electron flood gun mounted line of sight with sample
electron flood gun mounted in analyzer axis + electromagnet
Which way is B field?

36. Quantitative analysis
X-ray penetrate much deep than the escape depth of electrons

37. Can be found in
handbook

38. How to measure the intensity
Lorentzian or Gaussian functions plus a background (or even more complicated functions) of E can be used to fit the peak to subtract the background. (More complicated Shirely background.)

39. Instrumentation (analyzers)
Analyzer: most essential part of any electron spectroscopy, its characteristic are: energy range, energy resolution, sensitivity and acceptance angle. Normally its functions involve: retarding of the incoming electron, selection of the electrons with right kinetic energy (pass energy), detecting of the electrons (channeltron)
resolution
Acceptance angle

40. Hemispherical Analyzer
Outer Sphere
Inner Sphere
Analyzer Control
Electron
Optics
Multi-Channel channeltron Electron Multiplier
X-ray
Source
Most widely used for XPS
Sample

41. Hemispherical Analyzer
Pass energy: E = e U (b/a - a/b)
Resolution:
DE/E = (x1+x2)/2r +a2
a=(a+b)/2
U is the voltage difference between inner and outer sphere; a and b are radii of inner and outer spheres; x1 and x2 are the radii of the entrance and exits apertures, respectively; a is the maximum deviation of the electron trajectories at the entrance with respect to the center line.

42. Angular resolved XPS Why?
Photoemission is a dipole interaction, its Hamilton can be write as:
The transition possibility is:
with
Obviously the experimental geometry (the directions of the incident light and electron emission) is crucial to the photoemission process. Moreover, the electronic structure will be influenced by the presence of the surface, its possible influence will be present by the sample normal.
The change of emission angle with respect to the sample normal can also give different surface sensitivity.
The angular dependence of XPS is how the photoelectron diffraction (XPD) is done, which gives the structural information of the surface.

43. Angular resolved XPS
Various angular dependence

44. Surface sensitivity change due to angle and photon energy
More Surface Sensitive
less Surface Sensitive
Same path length but the depth different
Can be done aslo with AES!

45. Ekin = hn - EB For photoemission,
Sample for surface sensitivity change due to angle
Ekin = hn - EB
For photoemission,
Change of photon energy can change photoelectron energy that also changes the free path length of the photoelectrons(surface sensitivity).
Cannot be done with AES!

46. AES vs. XPS 1. Common points Both elemental and chemical sensitive
Both can be used to do quantitative analysis of chemical composition.
Both are electron spectroscopy which have surface sensitivity.
2. difference
AES: involved two electrons and one hole, due to coulomb interaction (no selection rule), complicated, peak broad, can be excited by many energetic particles including photon, no intrinsic angular dependence, commonly use CMA, AES peak in XPS spectra is with fixed Ekin.
XPS: involved in one electron, due to dipole interaction (selection rule), peak sharp, simple, only excited by photon, sensitive to angular geometry, often use angular resolved analyzer, XPS peak is with fixed binding energy.