Photoelectron Spectroscopy (PES)

You’ll be able to

Explain how PES uses photon energy and electron kinetic energy to measure binding energies
Read a PES spectrum: peak position as subshell binding energy, peak height as electron count
Match a PES spectrum to an element and its electron configuration

Shining light hard enough to knock electrons loose

Photoelectron spectroscopy fires high-energy photons (usually X-rays or UV) at a sample. If a photon carries more energy than an electron’s binding energy — the energy holding that electron to the atom — the electron is ejected. The instrument measures the ejected electron’s kinetic energy, and the rest is bookkeeping: whatever photon energy wasn’t spent escaping shows up as motion.

The PES energy balance

binding energy = photon energy − kinetic energy of ejected electron

Because every subshell holds its electrons with a characteristic tightness, each subshell produces its own peak at a fixed binding energy.

Reading the two axes: position and height

A PES spectrum plots binding energy on the horizontal axis (conventionally with higher binding energy on the left). Two features carry all the information. Peak position tells you which subshell: core electrons (like 1s) sit very close to the nucleus, feel almost the full nuclear charge, and appear at high binding energy; valence electrons are shielded and loosely held, appearing at low binding energy. Peak height (strictly, peak area) is proportional to the number of electrons in that subshell — a 2p⁶ peak is three times as tall as a 2s² or 1s² peak.

From spectrum to element

To identify an element, count the peaks (that is how many occupied subshells it has), read each peak’s relative height (electrons per subshell), and add them up. For a neutral atom the total equals the atomic number Z. The heights, read left to right, spell out the electron configuration directly.

Worked example

A PES spectrum shows three peaks. From highest to lowest binding energy, their relative heights are 2 : 2 : 5. Identify the element and write its configuration.

1.Three peaks means three occupied subshells. In order of decreasing binding energy these are 1s, 2s, 2p.
2.Peak height is proportional to the electron count in each subshell: 1s², 2s², 2p⁵.
3.Total electrons = 2 + 2 + 5 = 9, so Z = 9 for a neutral atom.
4.Z = 9 is fluorine, configuration 1s² 2s² 2p⁵.

Answer: Fluorine, 1s² 2s² 2p⁵

Checkpoint

In a PES spectrum, a peak at *higher* binding energy corresponds to electrons that are:

Tip

Don’t confuse the two axes. Position (left–right) answers “which subshell / how tightly held?” Height answers “how many electrons?” A tall peak on the far left is a fully populated core subshell, not a loosely bound one.

Checkpoint

A PES spectrum has four peaks whose relative heights, from highest to lowest binding energy, are 2 : 2 : 6 : 2. Which neutral atom is it?

On the exam

AP frequently shows two spectra and asks which belongs to the larger atom or which subshell was affected by ionization. Anchor every answer in the two rules: peak position = binding energy (tighter hold, higher Zₑff, shifts left), peak area = electron count. Removing an electron shrinks the corresponding peak’s height.

Answer the 2 checkpoints as you read.