Photoelectron spectroscopy, or PES, is a method used to study how electrons are arranged in atoms and ions. A sample is hit with high energy photons, and electrons are ejected from the atom. By measuring the kinetic energy of those electrons, scientists calculate how strongly each electron was held by the nucleus.
This matters because a PES spectrum gives direct evidence for electron shells, subshells, and electron configurations.
In a PES graph, the x-axis shows binding energy and the y-axis shows relative number of electrons. Peaks farther to the high binding energy side represent electrons that are harder to remove, usually inner-shell electrons closer to the nucleus. Taller peaks represent more electrons in that subshell, so peak height or area is related to subshell population.
By reading peak positions and relative peak sizes, you can identify which subshells are present and compare atoms across the periodic table.
Key Facts
- Binding energy is the energy needed to remove an electron from an atom or ion.
- PES uses the photoelectric relationship KE = hf - BE, so BE = hf - KE.
- Higher binding energy means the electron is more strongly attracted to the nucleus.
- Peak position on a PES spectrum identifies the subshell energy, such as 1s, 2s, or 2p.
- Peak height or peak area is proportional to the number of electrons in that subshell.
- Core electrons usually appear at higher binding energies than valence electrons.
Vocabulary
- Photoelectron spectroscopy
- Photoelectron spectroscopy is a technique that uses photons to eject electrons and measure their binding energies.
- Binding energy
- Binding energy is the minimum energy required to remove an electron from an atom or ion.
- Photoelectron
- A photoelectron is an electron ejected from an atom after it absorbs energy from a photon.
- Subshell
- A subshell is a group of orbitals with the same principal energy level and type, such as 2s or 2p.
- Effective nuclear charge
- Effective nuclear charge is the net positive pull felt by an electron after shielding by other electrons is considered.
Common Mistakes to Avoid
- Reading the binding energy axis backward, because many PES spectra place higher binding energy on the left and lower binding energy on the right.
- Assuming the tallest peak always has the highest binding energy, because peak height shows relative number of electrons, not how tightly they are held.
- Matching every peak to one orbital instead of one subshell, because a single PES peak usually represents all electrons in a subshell such as 2p.
- Ignoring shielding and effective nuclear charge, because electrons in the same shell can have different binding energies depending on subshell penetration and nuclear attraction.
Practice Questions
- 1 A photon with energy 125 MJ/mol ejects an electron with kinetic energy 20 MJ/mol. What is the binding energy of the electron?
- 2 A PES spectrum has three peaks with relative heights 2, 2, and 6 at increasing distance from the nucleus. What electron configuration is suggested for the atom?
- 3 Two atoms have valence 2p peaks, but atom B has its 2p peak at a higher binding energy than atom A. Explain what this suggests about the attraction between the nucleus and the 2p electrons in atom B.