Photoelectron spectroscopy

Photoelectron spectroscopy provides direct experimental evidence for the shell and subshell structure predicted by electron configurations. High-energy photons eject electrons from atoms, and the measured binding energies reveal how tightly each group of electrons is held by the nucleus.

Each peak in a PES spectrum

• Corresponds to a specific subshell
• Its position on the binding energy axis reflects the strength of the electrostatic attraction between those electrons and the nucleus
• Its relative height (intensity) indicates how many electrons occupy that subshell

Units Binding energy is conventionally reported in megajoules per mole (MJ/mol), and the axis is usually drawn with binding energy increasing toward the left, so the most tightly held, innermost electrons appear farthest left.

Interpreting a PES spectrum

• Requires reading from high to low binding energy, matching each successive peak to the subshells in the ground-state electron configuration (1s, 2s, 2p, 3s, and so on)
• Core electrons appear at the highest binding energies because they are closest to the nucleus and least shielded
• As nuclear charge increases across a period, all peaks shift to higher binding energies because the increased attraction holds the electrons more tightly
• Summing the relative heights of all peaks gives the total number of electrons, which directly identifies the element or ion

PES data therefore confirm that electrons exist in quantized energy levels and that shielding and nuclear charge govern how tightly each subshell's electrons are bound.