PHOTOELECTRON SPECTROSCOPY
Learning Objective: To understand the role, location, and nature of electrons in
an atom.
NC Standards: 2.06, 3.01
Pre-Requisite: An understanding of the atom – suggest The Atom Inquiry Activity
Skills: Data analysis
Strand: Nature of science, science as inquiry and science and
technology
Safety Precautions: None
Science Concepts: Understand the different types of orbitals, the
location of electrons, how many electrons are in
each orbital, how many orbitals are in each
sublevel, how ionization energy relates to electron
configuration and how ionization energy relates to
the distance of an atom from the nucleus.
Materials: No additional material needed
Background Information:
Earlier we proposed an atomic model with electrons in orbitals around the nucleus, some further away
from the nucleus than others. If an electron is said to occupy an energy level in an atom then each electron
must be in an orbital at a particular distance from the nucleus and the energy levels corresponding to these
orbitals have a certain energy level.
The next step in the location of the electron required Photoelectron Spectroscopy. In chemistry,
spectroscopy is a technique that is used to detect what type, and/or what amount, of a chemical substance
might be in a chemical sample. Spectroscopic methods are quite common and popular in areas such as
forensic (crime) science, where they are used to detect what might be in a murder victims blood, or what
might be in some substance found at a crime scene.
In photoelectron spectroscopy (PES), we can detect what substance we have by measuring the amount of
energy needed to remove electrons. All substances contain electrons. Electrons can be added to a
substance or removed from a substance. If we add an electron, the substance becomes negatively charged.
If we remove an electron, the substance becomes positively charged. We can write this last example as:
MM  e
This equation says that we have a molecule (M), and it loses an electron to become M+. How does this
happen? Energy is needed to remove the electron. This energy is called the ionization energy. When an
electron is removed, we say it is ionized, and the energy needed to do that is the ionization energy (IE).
Where does that energy come from? It can come from
light energy, which is called a photon. When energy
from a photon (shown in the graphic as squiqqly red
arrows) strikes the surface of a substance some of the
electrons “ionize”, and leave the substance. The
substance now has a positive charge. In photoelectron
spectroscopy, we are interested in measuring the
energy required to remove these electrons. It is often
the case that there is more energy striking the surface
than is actually needed to remove the electron(s).
When that happens, the electrons also have kinetic
energy.
The table below provides a short overview of the process:
Description
In the beginning condition, we have two
electrons that are bound to the molecule
(represented by the letter “M”. This is called
the initial state.
After energy strikes the metal surface, an
electron moves to the next level, called the
vacuum level. Given that the electron has
left a “hole” in the molecule, there is now a
plus charge on the molecule.
The energy required to move the electron
from the bound state to the vacuum level is
known as the ionization energy (I.E.). It is
possible, as is shown here, for the electron to
move past the vacuum level.
Assuming that the electron does move past
the vacuum level, the extra energy is in the
formed of kinetic energy (K.E.). The metal
is now M+ (a positively-charged metal) plus
an ionized, or free, electron.
Image
The photoelectron spectrum is a plot of the number of electrons emitted versus their kinetic energy. In
the diagram below, the “X” axis is labeled high to low energies so that you think about the XY intersect
as being the nucleus.

Orbital names s, p, d, and f stand for names given to groups of lines in the spectra of the alkali metals.
Early chemists called the line groups sharp, principal, diffuse, and fundamental.
Electron configuration: a notation describing electrons in an atom. For example
Sodium electrons are represented as1s22s22p63s1. The coefficient tell us the energy level and the
superscript tells us the number of electrons in that energy level.
Interpretations from the data:
1. There are no values on the y axis in the tables above. Using the Periodic Table and Table 1, put
numbers on the y axis.
2. Label each peak on the graphs above with s, p, d, or f to indicate the suborbital they represent..
3. What is the total number of electrons in a neutral potassium atom?
4. How does the number of electrons in a neutral potassium atom relate to the Atomic Number of
potassium?
5. Potassium loses one electron when forming an ion. Which electron is mostly to be removed to form K+?
_______________ Explain _______________________________________________________
_____________________________________________________________________________
6. Sketch a photoelectric spectrum of calcium. (Don’t try to make it to scale)
7. What is the trend in ionization energy for electrons as they move further from the nucleus?
8. Why is the ionization energy for the 1s orbital in calcium greater than the ionization energy of the 1s
orbital in potassium? __________________________________________________________________
9. What element could have the following spectrum? __________
10. Examine the graph below.
a) Which electron(s) is represented by the 0.77 peak?________________
b) Which electron(s) is represented by the 0.63 peak? _______________
c) Scandium looses two electrons when forming Sc2+. Which electrons are most likely to be
removed? __________________________.
Explain the logic for your answer. ____________________________________________
11. Draw a model of a scandium atom by sketching the relative distance of each energy sublevel
from the nucleus. Describe how your model compares to the spectrum in #6.
12. Using the Periodic Table, what is the relationship between sublevels and the number of orbitals
within each sublevel?
13. Sketch a spectrum of bromine ( label the “Y” axis quantitatively but not the “X” )
14. Write the electron configuration of the following.
a. K
b. K+
c. Ca
d. Ca2+
e. Sc
f. Sc2+
Teacher notes:
This activity should be done after “The Atom” activity. You should discuss the break //in the “X”
axis and why it is drawn this way.
The following is a more complete explanation of photoelectron spectroscopy. You way wish to us
with your honors or advanced students.
We can use mathematics to describe the concept above. We can write this equation:
IEE(h )KE(e )
This equation states that the ionization energy (IE) is equal to the amount of energy coming in (the photon energy,
represented by hv) minus the kinetic energy of the electron.
If we have a neutral (uncharged) molecule that is a gas, the photon (hv) ionizes the molecule (M), and leaves the
molecule in a positively charged ion state (Mi+). We also now have a free electron (e-):
M  photon(h ) Mi  e
We can rearrange the mathematics to show that the ionization energy is also a measure of the difference in energy
between the positive ion state Mi+ and the initial state of the molecule M.
IE  E(Mi ) E(M) E(h ) KE(e )
The difference between the energy found in the bound state (also known as the ground state) and the vacuum level
(also known as the excited state) is what we measure in photoelectron spectroscopy, or PES. Spectroscopy is the
technique in chemistry where we measure the interactions between light energy (photons) and some molecule or
molecular system.
We can measure ionization energies using a number of techniques, including molecular modeling. One of the
quantities that can be measured are the energies of the molecular orbitals, or MOs. Two of the MOs are of particular
interest: the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital
(LUMO). These orbitals are the most important in terms of chemical reactivity. If an electron in a molecule is
going to leave, it’s probably going to leave from the HOMO. If an electron is going to be added to a molecule, it’s
probably going to find its home in the LUMO. Collectively,
these orbitals are known as the frontier orbitals (because
they are on the “frontier” of chemical reactivity). More
importantly, Koopman’s Theorem states this: the value
of the HOMO is also the value of the ionization energy.
Thus, we can use molecular modeling to calculate the
HOMO, and from that, immediately derive the ionization
energy.
If the students color code the main energy levels in
question 11 it would reinforce the difference between
main energy levels and sublevels.
The use of a long piece of paper tape in question 13 would beneficial in helping the students better
understand the relative distances between sublevels.
Activity and spectrum graphics taken from the Pogil Project activities.
(Photoelective Graphics adapted from http://www.pes.arizona.edu/facility/aboutPES.htm, with permission
of the author. Photoelectric effect graphic from http://en.wikipedia.org/wiki/Photoelectric_effect)