Photoelectron Spectroscopy

Electrons in an atom sit in distinct shells and subshells, each at its own energy, not spread out continuously. When a photon hits an atom with enough energy, it knocks an electron loose, and the leftover energy becomes the electron's kinetic energy. Because every electron in a given subshell started at the same binding energy, they all leave with the same kinetic energy, so the detector sees a sharp spike rather than a smear. Different subshells need different amounts of energy to empty, so the spectrum shows a separate peak for each one.

To read a spectrum, look at two things on each peak: its position along the binding energy axis and its height. Position tells you how tightly that subshell's electrons are held — far from the nucleus and shielded means low binding energy, close to the nucleus means high binding energy. Height tells you how many electrons occupy that subshell, since more electrons at the same energy give a taller signal. Matching peaks to subshells means ordering them by energy (1s, then 2s, then 2p, and so on, from hardest to easiest to remove) and reading off the electron count from each height.

This turns PES into a direct experimental check on electron configuration: add up every peak height and you get the total electron count, which should match the atomic number for a neutral atom. Moving from neon to sodium, sodium's extra proton pulls all its electrons in a bit tighter, shifting every peak to higher binding energy, and a brand new small peak appears for the single 3s electron sodium has that neon does not.