PES
photoelectron spectroscopy
How does PES work?
PES works on the same principle as the
photoelectric effect.
Photoelectric Effect
of visible light
http://hyperphysics.phy-astr.gsu.edu/hbase/mod1.html
•The photon has energy larger than the binding energy (Eb) of the atoms specific electron.
•The photon typically used with PES is Uv light or x-ray.
•The ejection of an electron produces a positive ion.
Ultraviolet Photoelectron Spectroscopy
valence
electrons
core
electrons
Decreasing Binding Energy
UV (hn)
Ek = hn – Eb
Eb = hn – Ek
ejected valence electron (e-)
Ek = kinetic energy
Eb = binding energy
X-ray Photoelectron Spectroscopy
valence
electrons
X-ray (hn)
Eb = hn – Ek
ejected core electron (e-)
core
electrons
Decreasing Binding Energy
Ek = hn – Eb
Ek = kinetic energy
Eb = binding energy
Useful Unit Conversions
1 eV = 1.602 10
-19
J
1 eV = 9.648 10 J mol
4
1
1
10 eV = 9.648 10 J mol = 0.9648 MJ mol
5
-1
MJ = megajoule (this unit is a 1000x greater than kilo)
•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
• 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
• 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.
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.
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.
Hydrogen
• has 1 peak in the PES
scan above because it has 1
electron. (1s1)
•The peak measures 1.312 MJ/mol
(1312 KJ/mol)
•This is the energy required to eject
the electrons from one mole of
hydrogen atoms.
Helium
•has one peak (1s2)
•It is shifted to the left as
compared to hydrogen’s peak.
•The peak measures 2.372 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)
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)
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.
Boron
2:1 ratio of electrons
n=1
n=2
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)
• 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.
So our expectationIt 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.
Useful Web Sites
Simulated photoelectron spectra for some common elements
• http://www.chem.arizona.edu/chemt/Flash/photoelectron.
html
A complimentary lesson on PES
• http://www.bozemanscience.com/apchem-004-coulombs-law/
Sulfur 2p Binding Energies
Skoog, Holler, Crouch, “Principles of Instrumental Analysis,” 6th Ed. p
597.
Label the sublevels