Photoelectron Spectroscopy
Drill
• Use your own device to look up the vocab
terms to help you answer this question.
Answer
What is Spectroscopy?
• Spectroscopy pertains to the dispersion of an
object's light into its component colors
(i.e. energies).
Definition
• Photoelectron spectroscopy (PES) is a
technique used for determining the ionization
potentials of molecules.
Question
• Do all of the electrons in a given shell
have the same energy?
• This question can be answered by using PES to
measure the energy required to remove an
electron from a neutral atom in the gas phase
to form a positively charged ion.
What PES Can Do
• PES can remove electrons from any shell in the
atom. (valence or core electron)
• Only a single electron is removed from a given
atom, but that electron can come from any
energy level.
• PES allows us to measure the energy needed
to remove any electron in an atom.
PES Tutorials
• http://www.sophia.org/tutorials/apchemistry-pes
• or
• http://youtu.be/tpNbBV7Hk6k
What Happens During PES…
• PES shines radiation on a sample.
The radiation will have enough energy to
excite an atom to the point that
an electron can be ejected from any shell of the atom
to form a positively charged ion.
• The K.E. of the ejected electron is measured, and the
energy required to remove the electron from the atom
is calculated from the difference between the energy
of the photon (hv) and the K.E. of the photoelectron.
Spencer, p. 88-89
What Happens During PES…
• A PES experiment begins with the absorption of a
high-energy UV or X-ray photon that carries more
energy than the I.E. of the atom.
• The excess energy is carried off by the electron
ejected from the atom in the form a K.E.
• The energy of the photon (hv) absorbed by the
atom is therefore equal to the sum of the I.E. of
the atoms and the K.E. of the electron that is
ejected from the atom.
hv = I.E. + K.E.
Spencer, p. 88-89
Calculation
Kinetic energy of the
photoelectron ejected
when the radiation is
absorbed
Ionization Energy
K.E. = hv - B.E.
Energy of the radiation
absorbed by the atom
I.E. can also be called Binding Energy.
This is the energy required to either separate an electron from an atom or to separate the
protons and neutrons of an atomic nucleus.
Equipment Diagram
http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Photoelectron_Spectroscopy/Photoelectron_Spectroscopy%3A_Application#Detection_.26_Spectra
Detection
• Detection relies on the ability of the instrument to
measure energy and photoelectron output. One type
of energy measured is the binding energy, which is
calculated through the following equation:
where:
• KE = hν – BE
Ke = Kinetic energy, this is
measured hv = Photon energy from the radiation
source, this is
controlled by the source BE= Binding energy,
this is the unknown of interest
and can be calculated from the other
variables
http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Photoelectron_Spectroscopy/Photoelectron_Spectroscopy%3A_Application#Detection_.26_Spectra
Worksheet
• Distribute the Photoelectron Spectroscopy WS
to use for discussion.
Spectral Output
• The spectrum produced from a
PES experiment has peaks that correspond to
the ionization potentials of the
molecule. These also correspond to the
orbital energies.
http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Photoelectron_Spectroscopy/Photoelectron_Spectroscopy%3A_Application#Detection_.26_Spectra
PES Data
• Data from PES experiments are obtained as
peaks in a spectrum that plots the intensity of
the observed signal on the vertical axis versus
the energy needed to eject an electron (IE) on
the horizontal axis.
Spencer, p. 89
PES Data
• The spectrum is plotted so that energy
increases from left to right on the horizontal
axis.
• The height of the peak in the PES spectra is
directly proportional to the number of
electrons of equivalent energy ejected during
the experiment.
Spencer, p. -89-91
PES Data
• If you see two peaks that have a relative
height ratio of 2:1, it may be concluded that
one of the energy levels (from which electrons
are removed) contains twice as many
electrons as the other.
Spencer, p. -89-91
Spectra
• The following slides contain spectra generated
from the following website:
• http://www.chem.arizona.edu/chemt/Flash/p
hotoelectron.html
• http://www.pes.arizona.edu/aboutPES.htm
• http://www.sisweb.com/mstools/isotope.htm
Reading the Spectra
• The data is graphed as spectrum of
photoelectron counts versus kinetic energy
Hydrogen has only one electron,
therefore only one peak is shown.
Notice that the peak for He is located farther to the right
of the peak for H and the height of the peak is approximately
twice the height of H.
(There is only one peak because He only has one orbital shell.
http://www.chem.arizona.edu/chemt/Flash/photoelectron.html
Notice that there are 2 peaks shown for Li
that correspond to the two orbital shells.
1s level
2s level
1s level
As you read one of these diagrams remember to think about which energy level
should take more energy to remove an electron.
http://www.chem.arizona.edu/chemt/Flash/photoelectron.html
Notice how large the energy value is for removal of the
second electron (on both spectra) compared to removal
of the first electron. (Remember that the 2nd e- is from an inner shell.
X axis scale change
http://www.chem.arizona.edu/chemt/Flash/photoelectron.html
Just a Note
• The PES spectra for B, C, N, O, F and Ne
contain a second peak, of gradually increasing
energy because of the increasing nuclear
charge, that has the same intensity as the
peak for the n=1 shell.
• In each case there is a third peak (of gradually
increasing energy) that corresponds to the
electrons that are the easiest to remove from
the atoms.
Spencer, p.91
http://www.chem.arizona.edu/chemt/Flash/photoelectron.html
It’s Interactive. Give it a try.
Learn More About Spectroscopy
• http://www.chem.qmul.ac.uk/surfaces/scc/sc
at5_3.htm
• http://www.rsc.org/learnchemistry/collections/spectroscopy/introducti
on#
More Info
• The following website offers more background
information about PES
• http://www.chem.qmul.ac.uk/surfaces/scc/sc
at5_3.htm
Additional Info
• I found a guided inquiry activity about
Photoelectron Spectroscopy in
Moog's Chemistry; a Guided Inquiry; 4th
edition.
• http://www.gobookee.net/chemistry-aguided-inquiry-4th-edition-solutions/
Resources
• Spencer, J. N., Bodner, G. M., Rickard, L.H., (2010).
Chemistry: Structure and dynamics. (5th ed.). Wiley.
• University of Arizona Department of Chemistry and
Biochemistry. (2013) Photoelectron Spectra. Retrieved
from
http://www.chem.arizona.edu/chemt/Flash/photoelec
tron.html
• http://bit.ly/1195Ahl
• http://www.chem.qmul.ac.uk/surfaces/scc/scat5_3.ht
m