Photoelectron Spectroscopy

Learning Objective 1.6.A Explain the relationship between the photoelectron spectrum of an atom or ion and: i. The electron configuration of the species. ii. The interactions between the electrons and the nucleus.

Quick Notes

Photoelectron Spectroscopy (PES) measures the energy needed to remove electrons from different subshells of an atom.
A PES spectrum shows:
- x-axis = binding energy (energy needed to remove electron)
- y-axis = relative number of electrons (peak height)
Each peak corresponds to a subshell (e.g. 1s, 2s, 2p).
Peak position (along x-axis) shows how tightly electrons are held — closer to nucleus = higher binding energy.
Peak height = number of electrons in that subshell.
PES data supports and reflects the atom’s electron configuration.

Full Notes

Photoelectron Spectroscopy (PES) is a powerful experimental technique used to investigate the electronic structure of atoms. It allows us to determine how much energy is needed to remove electrons from different energy levels (or subshells) within an atom or ion.

What Does PES Measure?

PES measures the binding energy of electrons – the energy required to remove an electron from a specific orbital.

Electrons that are closer to the nucleus are more strongly attracted and require more energy to remove.
The data from PES is presented as a photoelectron spectrum.

Interpreting a PES Spectrum

x-axis: binding energy (in megajoules per mole, MJ/mol or electronvolts, eV)
y-axis: relative number of electrons (peak height)
Each peak represents electrons in a specific subshell. The height of the peak shows how many electrons are in that subshell. The position of the peak (how far to the right on the x-axis) shows how strongly the electrons are attracted to the nucleus of the atom.

AP Chemistry PES example spectrum with labelled 1s, 2s and 2p peaks; heights show electron counts and positions show binding energy

Relationship to Electron Configuration

PES spectra can be directly linked to electron configurations. Each group of peaks matches the electrons in different subshells.

Example: A PES spectrum for oxygen (1s² 2s² 2p⁴) would show:

One peak at high binding energy → 1s (2 electrons)
One peak at lower energy → 2s (2 electrons)
One larger peak at slightly lower energy → 2p (4 electrons)

The relative heights reflect the number of electrons in each sublevel, and the binding energies reflect how close those electrons are to the nucleus.

Interactions Between Electrons and the Nucleus

Electrons in inner shells are closer to the positively charged nucleus and are more strongly attracted, which means they require more energy to remove.
The farther an electron is from the nucleus, the lower its binding energy.
Shielding from inner electrons reduces the attraction felt by outer electrons, lowering their binding energy.

These effects are visible in a PES spectrum, where:

Inner electrons (like 1s) show peaks at high binding energy
Outer electrons (like 2p or 3p) show peaks at lower binding energy

Worked Example

Element X has a PES spectrum with the following features:

One large peak at high binding energy
One medium peak at intermediate energy
One medium peak at low binding energy

Interpretation:

AP Chemistry PES spectrum of fluorine with 1s, 2s and 2p peaks annotated and electron configuration 1s2 2s2 2p5

High energy peak (2 electrons) = 1s
Intermediate peak (2 electrons) = 2s
Low energy peak (5 electrons) = 2p → Electron configuration = 1s² 2s² 2p⁵ → Element F is fluorine

Matt’s exam tip

In PES questions, be ready to match spectra to electron configurations or explain differences in binding energy. Focus on how close electrons are to the nucleus and how many occupy each subshell. Also, watch the x-axis as binding energy often decreases to the right, which can trip students up.

Summary

Photoelectron spectroscopy (PES) provides experimental evidence for the electron configuration of atoms.
The position of each peak reflects how tightly an electron is held (binding energy), and the height corresponds to the number of electrons in that subshell.
Binding energy increases for electrons closer to the nucleus; shielding lowers outer‑electron binding energies.
PES supports the Aufbau principle and electron configurations.