Photoelectron spectroscopy

Interpreting Photoelectron Spectra

Photoelectron spectroscopy (PES) provides direct experimental evidence for the subshell structure of atoms by measuring the energy required to remove electrons from each subshell.

In a PES experiment, high-energy photons (usually X-ray or ultraviolet) strike a gaseous sample and eject electrons from atoms. The kinetic energy of each ejected electron is measured, and because the photon energy is known, the binding energy [the energy required to completely remove an electron from its subshell] can be calculated.

Electrons closer to the nucleus experience a stronger electrostatic attraction and therefore require more energy to remove — they appear at higher binding energies on the spectrum. Electrons in outer subshells are shielded from the full nuclear charge by inner electrons, so they have lower binding energies.

A PES spectrum plots binding energy on the x-axis (typically decreasing from left to right) against relative number of electrons on the y-axis (signal intensity). Each peak corresponds to a specific subshell: peaks at high binding energy correspond to core electrons (e.g., 1s), while peaks at low binding energy correspond to valence electrons.

The relative height of each peak is ideally proportional to the number of electrons in that subshell, so a peak twice as tall as another contains twice as many electrons.

A PES spectrum can be read as a direct map of the ground-state electron configuration. For example, nitrogen (1s² 2s² 2p³) produces three peaks:

• the leftmost (highest binding energy) peak has a relative height of 2 (the 1s electrons)
• the middle peak has a relative height of 2 (the 2s electrons)
• the rightmost (lowest binding energy) peak has a relative height of 3 (the 2p electrons)

Moving across a period, the binding energy of every subshell increases because the nuclear charge increases, pulling all electrons closer and binding them more tightly. Moving to a new shell, however, produces a peak at noticeably lower binding energy because those electrons are farther from the nucleus and more shielded.

MisconceptionStudents often assume that taller peaks mean higher binding energy. Peak height represents the number of electrons in a subshell, not how tightly they are held. Position on the x-axis indicates binding energy.

Examiner InsightAP free-response questions frequently ask students to identify an element from its PES spectrum by counting total electrons (sum of all peak heights) and matching peak patterns to a ground-state electron configuration.