In this video, you will learn about the useful lab technique Photoelectron Spectroscopy (PES). Additionally, you will study graphs made from PES data and interpret their meaning to ultimately understand how data from PES can be used to determine electron configurations and describe atomic structure.
What Is Photoelectron Spectroscopy?
Photoelectron Spectroscopy. Do any parts of this term remind you of anything?
- Photo could have to do with light, radiation, photons, or maybe even an image of something.
- Electrons are the tiny negatively charged particles buzzing around the nucleus of an atom.
- Spectroscopy is any lab technique that uses radiation to analyze matter.
Put all these terms together, and we have an idea of what photoelectron spectroscopy is all about!Photoelectron spectroscopy, usually abbreviated PES, is a lab technique that involves shooting an intense beam of radiation at a sample of an element and measuring the energy of the electrons that are ejected from the sample. While this technique definitely sounds a little science-fiction-y, it is very useful. Data from PES can be used in a variety of ways, from determining electron configurations to better understanding atomic structure. But more on that later. For now, let’s go into lab and check out our PES instrument.
In this instrument, a beam of radiation (either ultraviolet or X-ray) is directed at a sample of an element.
Ideally, it’s a pure sample. Energy from the intense radiation causes electrons from the sample to pop off and fly away. These electrons hit an electron detector that measures the kinetic energy of the electrons. Kinetic energy is energy in motion. It is usually recorded in electron volts (eV), a unit that measures electron energy. Electron energies are recorded and then analyzed.
At the end of data collection, scientists know two things: the energy of the radiation going into the experiment and the kinetic energy of the ejected electrons. From this data, the binding energy, or the energy it takes to remove an electron from an atom, can be determined. Electron binding energy is very closely related to the concept of ionization energy, the energy needed to remove an electron from an atom in a gaseous state.
It is okay to think of binding energy as ionization energy.To do so, we use the equation:E (photon) = Kinetic Energy (KE) + Binding Energy (BE)Energy of radiation equals the kinetic energy of the electron plus the binding energy.Energy of the radiation can be calculated from the formula:E (photon) = hvh = Planck’s constant (6.362 x 10^-34 Js)v = frequency of the wavelengthWhen radiation of consistent energy is used, electrons with higher binding energies will register with less kinetic energy.
Electrons with lower binding energies will register with more kinetic energy. A graph is then made of the binding energy data for individual elements.
Graphs Made from PES
Unless you are in upper-level college chemistry courses, the graphs you will see for PES are idealized graphs, so a lot of the background noise has been removed.What we get to see are graphs plotting signal intensity, often reported as relative number of electrons, against binding energy of electrons.
Sometimes, like in the graph shown here, there are no units – the title is simply ‘Energy.’
Other times, the units may be in terms of megajoules per mole (MJ/mol) or electron volts (eV). Peaks showing electron energies are sometimes even labeled with the relative number of electrons responsible for the signal!Before we dig into this graph for lithium, let’s think for a minute about what we know about electron configurations.
We know that lithium has three electrons in a 1s^2 2s^1 configuration. This means that there are two core electrons very close to the nucleus and that there is one valence electron that spends most of its time farther away from the nucleus. Of these electrons, which one is held the tightest by the nucleus? Which one (or ones) should be more difficult to remove?It’s pretty easy to remove that first electron. It has a low binding energy compared to the core electrons. On our graph of lithium, we see that there is a signal at a very low energy of 0.
52 that is approximately half of the height of the other signal, which registers at 6.26.
Notice that the x-axis is descending. This is meant to help you, as the student, visualize where the electrons are with respect to the nucleus.
Due to everything you learned in this lesson, you could subsequently:
- Define photoelectron spectroscopy (PES) by dissecting the term
- Understand graphs made from PES
- Describe real-world applications of PES