1.6 Photoelectron Spectroscopy

What PES measures

Photoelectron spectroscopy (PES) measures the energy needed to remove electrons from an atom. High-energy photons eject electrons; the binding energy of each electron is calculated from the kinetic energy lost. A spectrum plots binding energy (x-axis, decreasing left to right) vs. relative number of electrons (y-axis), revealing electron energy levels.

Reading a PES spectrum

Each peak represents a subshell of electrons at a distinct energy level. Peak height (relative intensity) is proportional to the number of electrons in that subshell. Peaks closer to the y-axis (low binding energy) are valence electrons, farther peaks (high binding energy) are core electrons, which are held more tightly by the nucleus.

Trends and evidence

Binding energy increases with nuclear charge (more protons pull electrons in tighter) and decreases with shielding/distance from the nucleus. PES data provide direct experimental evidence for electron configurations, subshell groupings (s, p, d), and periodic trends like increasing effective nuclear charge across a period.

Key terms

Photoelectron spectroscopy (PES)

A technique that measures the energy required to remove electrons from an atom, revealing subshell energies and electron counts.

Binding energy

The energy needed to remove an electron from a particular subshell; plotted on the x-axis of a PES spectrum, decreasing left to right.

Peak height in PES

Proportional to the relative number of electrons in that energy sublevel.

Core vs. valence electrons in PES

Core electrons show high binding energy (far from y-axis); valence electrons show low binding energy (closer to y-axis).

Why binding energy increases across a period

Effective nuclear charge increases as protons are added, pulling electrons closer and increasing the energy needed to remove them.

Try an AP-style question

Which element is it?

1. Lithium
2. Neon
3. Sodium
4. Magnesium