Photoelectron Spectroscopy (PES)

What the experiment does

A beam of energetic photons ejects electrons from occupied subshells. The electron kinetic energy and the known photon energy give the electron binding energy, E_binding = hν − E_kinetic. A PES spectrum plots signal against binding energy, with distinct features associated with occupied subshells. Check which direction the binding-energy axis runs before assigning inner and outer electrons.

Peak position: binding energy

Electrons closer to the nucleus are held more tightly, so core electrons appear at very high binding energy and valence electrons at low binding energy. For magnesium the peaks in order of decreasing binding energy are 1s, then 2s, then 2p, then 3s.

Peak area and electron population

The integrated area under a properly calibrated peak relates to the number of electrons in that subshell. Introductory diagrams often draw peaks with heights proportional to electron counts; follow the convention stated on the graph. For sulfur, [Ne] 3s² 3p⁴, a simplified spectrum shows a 3p-to-3s population ratio of 4:2, or 2:1. A real narrow peak can be taller than a broad peak yet have less total area, so height and area are not interchangeable without that schematic convention.

S: 1s² 2s² 2p⁶ 3s² 3p⁴

Counting peaks

The number of peaks equals the number of occupied subshells, not shells. Neon (1s² 2s² 2p⁶) has three peaks, with 1s at the highest binding energy. Magnesium (1s² 2s² 2p⁶ 3s²) has four peaks.

Comparing elements

The 1s peak of oxygen sits at a much higher binding energy than the 1s peak of carbon, because oxygen has 8 protons pulling on its 1s electrons versus 6 for carbon, with essentially no shielding between them. Same subshell, more protons, deeper binding.

Read a spectrum in three passes

First inspect the binding-energy axis: some PES graphs put high binding energy on the left, others on the right. Next identify occupied subshells from distinct peaks and compare their binding energies; inner-shell electrons usually require more energy to remove. Finally compare each peak's integrated area (or the relative magnitude shown in a simplified classroom spectrum) to estimate how many electrons it represents. A tall, narrow peak need not have more total electrons than a shorter, broad one if the graph is an experimental trace. Use the stated intensity convention. For a 2:2:6:2:1 subshell pattern, the total is 13 electrons, identifying neutral Al.

Keep peak height, area, and energy distinct

In a PES spectrum, a peak's area represents the number of electrons in a subshell; its binding-energy position reflects how tightly those electrons are held. Axes may run in either direction, so read the labeled energy scale before saying left or right. The energy to remove one outermost electron corresponds to the lowest-binding-energy electrons, while core peaks require much more energy.