PHOTOELECTRON SPECTOSCOPY (PES)
Teacher Guide
Organization: Small groups 2-4 students
Preparation: Several PES patterns illustrated on long paper (adding machine paper).
X vs. Y (energy vs. # electrons)
1
1.13
Hydrogen: Scale 0
10
2
0.9
Beryllium: Scale 0
10
2
11.5
10
100
6
2.08
2
4.68
Neon: Scale 0
10
2
84.0
10
100
Ask the student to examine the samples (experimental data) for the PES of the given elements,
answering and thinking about the following:
What? List facts about each element and the diagram/patterns for the elements
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The x-axis is energy (work, K.E., P.E.?)
The y-axis is number of electrons
These are supposed to be analysis of neutral atoms
Hydrogen has one proton and one electron and only one electron is accounted for
Beryllium has four protons and four electrons and all of the electrons are accounted for
Neon has ten protons and ten electrons and all of the electrons are accounted for
The pattern appears to be related to the common electron configuration pattern that is learned
in chemistry
The scale is weird 0 to 10 then 10 to 100 (log (base 10) scale)
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On the 0 to 10 scale is take less energy to remove the 6 electrons for Neon than the 2 electrons
on that same scale
The Hydrogen only has one electron and the scale is 0 10 10.
The Beryllium has 2 electrons on the 0 to 10 scale, but Neon has a total of 8 electrons on the 0
to 10 scale
The Beryllium has 2 electrons on the 0 to 100 scale and the Neon has 2 electrons on the 0 to 100
but the amount of energy is about 7/8 times larger
So What?
Develop any ideas about relationships or possible reasons for the diagram/pattern for each element.
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The number of electrons that are removed by the energy is the same as the number of total
electrons in the neutral atom of each of these species
The amount of energy needed to remove electrons varies based on the position of the electron
around the nucleus
The greater the nuclear charge (# protons) the larger the amount of energy to remove an
electron
These numbers somehow represent the amount of energy to remove an electron from an atom
electron to be removed The valence electrons require less energy to be removed
The electron configuration of an atom is directly related to this pattern in number of electron is
a specific energy level and sublevel
Sublevel within principle energy levels are the same energy position, so it take the same amount
of energy to remove electrons in the same sublevel and energy level
It takes more energy (10 to 100 times more energy to remove the inner most electrons
Electron configurations are represent theoretically calculated positions where an electron is
most likely to spin, these PES data analysis support that theory
What’s Next?
Ask the students to list some ways this information could be used.
Give the students actual data or send them to the website to investigate additional elements:
http://www.chem.arizona.edu/chemt/Flash/photoelectron.html
Put these questions on the unit exams
Have student identify an element based on the PES data given
Have a student predict the PES data for an element, based on this research
Additional topics to bring into this discussion:
Coulomb’s Law and formula
Photoelectric Effect (How Is this similar and different?)
Ionization Energy (How Is this similar and different?)
Explain the following relationship
-- Energy of Photon = Energy to Eject Electron + Work Function + Kinetic Energy
or
Energy to Eject Electron = Energy of Photon - Work Function – Kinetic Energy
Note: the work function is minimal in the gas phase (significant in the crystal phase).
Since the atoms are sublimed, the energy (x-axis) required to remove any specific electron from the
neutral atom is the same.
Also the energy to eject the electron is not a true I.E. but includes both the energy for one specific from
the neutral atom and the energy required to eject the electron from a specific position (such as 1s or 2s
or 2p or 3d or etc ) around the nucleus as well as move it away (Kinetic Energy)
Tricky Part: The valence electrons require the lowest amount of energy to be removed and move to
sensor, so the larger energy values are for the last electron in the electron configuration. These are
the electrons which already have the greatest P.E.(greatest position from the nucleus) and therefore
require the least amount of energy to move away. The core electrons (1s2) always require the most
energy to remove and the greater the nuclear charge (#protons) the larger the amount of energy to
remove the electron(s).
This is ratio stuff not necessarily exact data
Dr. Herman Krueger
PHOTOELECTRON SPECTROSCOPY (PES)
PRINCIPLES
--The name is suggestive of the phenomenon. Light (“photo-“) produces electrons. A spectrum of
energies needed to eject electrons will be surveyed.
--Photons of light carry a certain amount of energy that increases with the frequency of the light
(decreases with the wavelength of the light).
--When an electron absorbs (a photon of) light, the energy is used to:
a. Pull the electron off of the atom (“ionization energy”). This touches on the concepts of potential
energy and Coulomb’s Law.
b. Pull the electron away from surrounding atom and molecules (the “work function”). (This will be
minimal in the gas phase.)
c. Any left-over energy will appear as kinetic energy, which carries the electron to the detector.
-- Energy of Photon = Energy to Eject Electron + Work Function + Kinetic Energy
or
Energy to Eject Electron = Energy of Photon - Work Function – Kinetic Energy
All of the terms on the right side of the equation can be measured in the experiment. A spectrum thus
consists of a plot of “Electron Signal Intensity” (y-axis) vs. Energy to Eject Electron
THE PES SPECTRUM
--The spectrum consists of a series of peaks, implying that different sets of electrons in the atom have
different (well defined) energies.
--The height of the peaks can be used to determine the number of electrons in each set.
-- The x-axis of the spectrum represents how tightly the electron is bound, how hard one has to “pull” to
remove the electron found. The closer a set of electrons is to the nucleus, the more energy one needs
to eject the electron. Thus (this may be confusing for students) the farther a peak is from the origin
(from “zero” energy) the closer ( on the average) the electron set is to the nucleus.
--Note that the peak closest to the origin (by “the origin” I mean zero energy) goes with the outermost
electron. For a gas phase sample, the energy for this peak should correspond to the first ionization
energy.
--From the PES spectrum, we thus discover the shell structure of the atom along with the number of
electrons occupying each shell.
ACTIVITIES
--How to reinforce the somewhat “backwards” relationship that the electrons appearing at the highest
energy in the spectrum are actually the ones closest to the nucleus? One way of illustrating the concept
might involve lifting weights from various shelves and placing them on the top of the bookcase. The
weights are the electrons. When a weight is on top of the bookcase, it is an ionized electron—it is
detected. It takes more work (energy) to lift the weights from the lower shelves to the top of the case,
just as it takes more energy to ionize the electrons closest to the nucleus. The largest energy
expenditure goes with the weight closest to the floor. If the weights are a bit heavy, this might help the
students to “feel” (and remember??) the relationship between position and amount of energy
expended.
--An obvious activity would involve supplying students with spectra of “unknown” elements and
identifying the element. This simply involves counting electrons. To make the activity more challenging,
the students could be asked to write out the electron configuration for the atom, and specify the
relative (average) distances of each subshell from the nucleus.
Spectra for the various elements can be found at:
http://www.chem.arizona.edu/chemt/Flash/photoelectron.html
--Students can also be given an element and asked to sketch its (qualitative) spectrum.
MORE CHALLENGING ACTIVITIES
--Supply students with spectra for the elements in a given row of the periodic table. Focus on a given
subshell (say the 2s or 2p). Have students explain the change in peak position when progressing from
one element to the next. To explain the results, they would need to appreciate the concepts of
electron-electron repulsion and effective nuclear charge.
--Have students compute the energy difference between various subshells in an atom. This would
require them to recall what type of energy transition is involved in producing a given peak.
--Compare the position of the lowest energy peak (outermost electron) for elements in the same column
of the periodic table. This will reveal the trend for ionization energy as one goes down a column.
Students can use the concepts of potential energy and Coulomb’s Law to explain the result.