1. photoelectron spectroscopy
PES
photoelectron spectroscopy

2. How does PES work?
PES works on the same principle as the
photoelectric effect.

3. Photoelectric Effect of visible light

5. The photon has energy larger than the binding energy (Eb) of the atoms specific electron.
The photon typically used with PES is
The ejection of an electron produces a positive ion.
Uv light or x-ray.

6. Ultraviolet Photoelectron Spectroscopy
UV (hn)
Ek = hn – Eb
Eb = hn – Ek
valence
electrons
ejected valence electron (e-)
Decreasing Binding Energy
electrons
core
Ek = kinetic energy
Eb = binding energy

7. X-ray Photoelectron Spectroscopy
Ek = hn – Eb
valence
electrons
Eb = hn – Ek
X-ray (hn)
ejected core electron (e-)
Decreasing Binding Energy
electrons
core
Ek = kinetic energy
Eb = binding energy

8. Useful Unit Conversions
MJ = megajoule (this unit is a 1000x greater than kilo)

9. KE of the ejected photoelectron
The excess energy is carried off by the electron ejected in the form
of kinetic energy
The binding energy (energy required to remove this electron) from the atom is equal to the difference between the energy of the radiation absorbed by the atom (hv) and the Ek of the ejected photoelectrons.
Eb = hv – Ek
Absorbed energy
from the photon
KE of the ejected photoelectron

10. The values of binding energy found reveal information
about the elements present and the orbitals from which the
electrons were ejected
Lower binding energies are seen for valence electrons,
higher binding energies are seen for core electrons
For molecules, correlation tables are available to identify
the atomic composition of a molecule based on the binding
energies of the photoelectrons observed

11. For molecules, correlation tables are available to identify
the atomic composition of a molecule based on the binding energies of the photoelectrons observed
Skoog, Holler, Crouch, “Principles of Instrumental Analysis,” 6th Ed. p 593.

12. Data is obtained as peaks in a spectrum.
Two common ways to label the x-axis
The spectrum can be plotted so that energy increases toward the left
(typically) or right, on the x-axis.
Peak height is directly proportional to the number of electrons of equivalent
energy ejected.

13. the conclusion is the taller peak contains 2x the number of electrons.
If you see two peaks with a relative height difference of 2:1
the conclusion is the taller peak contains 2x the number of
electrons.

14. Hydrogen
has 1 peak in the PES
scan above because it has 1
electron. (1s1)
The peak measures MJ/mol
(1312 KJ/mol)
This is the energy required to eject
the electrons from one mole of
hydrogen atoms.

15. Helium
has one peak (1s2)
It is shifted to the left as
compared to hydrogen’s peak.
The peak measures MJ/mol
It takes more energy to remove an electron in the He atom vs H atom.
The peak is 2x that of hydrogen …..consistent with 2e- in He
(both H and He have electrons in n=1)

16. Lithium
has 2 peaks (1s2 2s1)
The smaller peak represents the 2s1
electron. It requires 0.52 MJ/mole to
remove one mole of electrons.
The larger peak represents the 1s2 core
electrons. It is twice the size as the
smaller peak
(correlating with the concept that twice the
peak size, twice the number of electrons).
Both electrons require 6.26MJ/mol energy
to be released. Energy is not represented
by the peak size. It relates to the electron
location within the atom. These electrons
are closer to the nucleus (core electrons)

17. Beryllium
has 2 peaks (1s2 2s2)
1:1 ratio both are of similar size
The peak to the right is of lesser energy
0.90 MJ/mol(valence shell electrons
n=2) compared to the inner peak
measuring 11.5 MJ/mol
(core electrons n=1). Once again the
peaks are similar in size
(both contain two electrons). But the
energy varies due to location within
the atom.

18. Boron
2:1 ratio of electrons
n=1
n=2

19. PES summary
PES determines the energy needed to eject electrons Which
is called the binding energy (ionization energy).
The PES data reinforces the shell model of the atom and infers the electronic structure of atoms.
PES scans support the idea that electrons in n=2 occupy different subshells (s, p), n=3 (s, p, d) and n=4 (s, p, d, f)

20. The peaks in n=1 and n=2 (He and Be respectively) are similar
in size proving 2 electrons are present in the two different PELS.
The difference in binding energy is a function of nuclear charge.
It takes more energy to remove an electron from n=2 than n=3.
And even more energy from n=1.
Within any subshell it always takes the most energy to remove an
electron from the s subshell.

21. So our expectation-
It takes more energy to remove an electron from n=2 than n=3.
And even more energy from n=1
Q: Do all electrons in a given shell have the same energy? A:
No…Electrons within a specific sublevel (s,p,d,f)
have similar energies.

22. Useful Web Sites
Simulated photoelectron spectra for some common elements
A complimentary lesson on PES

23. Sulfur 2p Binding Energies
Skoog, Holler, Crouch, “Principles of Instrumental Analysis,” 6th Ed. p 597.

24. Label the sublevels