1.6 PES

Photoelectron Spectroscopy (PES)

A log-scale spectrum on the exam's own 1000 to 0.1 MJ/mol axis for H through Ca. Hover a peak to link it to its subshell, shell and Coulombic pull, or overlay the neighbouring element.

Log-Scale SpectrumElectron ConfigCoulomb's LawH to Ca
Topic 1.6

Photoelectron Spectroscopy

Explain the relationship between the photoelectron spectrum of an atom or ion and (i) the ground-state electron configuration of the species, and (ii) the interactions between the electrons and the nucleus.

Photoelectron spectroscopy (PES) is the experiment that makes electron configuration visible. High-energy photons strike a sample and eject electrons; the instrument measures the kinetic energy of each ejected electron and works backward to its binding energy — the energy that was holding it to the atom.

EK 1.6.A.1 gives you exactly two rules for reading the spectrum:

  • Peak position = the energy required to remove an electron from that subshell. One peak per subshell.
  • Peak height = (ideally) proportional to the number of electrons in that subshell.

So a spectrum with peaks in the height ratio 2 : 2 : 4 must be 1s² 2s² 2p⁴ — eight electrons, oxygen. The configuration falls out of the picture.

Position is governed by effective nuclear charge. Core electrons sit close to the nucleus, are barely shielded, and have enormous binding energies. Valence electrons are farther out and shielded by every inner shell, so they come off far more easily. Within a single shell, s electrons penetrate closer to the nucleus than p electrons, so 2s binds slightly more tightly than 2p.

Comparing two elements' spectra is a direct probe of Zeff: the 1s peak of nitrogen sits at higher binding energy than the 1s peak of carbon because nitrogen has one more proton pulling on essentially the same core.

Key points

  • Peak height counts electrons; peak position measures how tightly they are held.
  • Higher binding energy = closer to the nucleus and less shielded = core electron.
  • Adding protons across a period raises Zeff and shifts every peak to higher binding energy.

Equations

  • not on the sheetConservation of energy applied to the photoelectric process. Not on the sheet, but the reasoning is expected.
    • binding energy of the ejected electron
    • energy of the incoming photon (hν)
    • measured kinetic energy of the ejected electron

Common mistakes

  • Peak height is an electron count, not an abundance. PES is not mass spectrometry — do not confuse the two graphs.
  • Axes are often plotted with binding energy decreasing to the right. Always read the axis labels before deciding which peak is the core.
  • A larger number of peaks means more subshells, not more electrons. Neon (1s²2s²2p⁶) has 3 peaks and 10 electrons; lithium has 2 peaks and 3 electrons.
  • Shielding is by inner shells, mostly. Electrons in the same shell shield each other only weakly, which is exactly why Zeff rises across a period.

Worked example

A PES spectrum shows peaks at 104, 6.84, and 3.67 MJ/mol with relative heights 2, 2, and 1. Identify the element and justify your answer.

Electron count: 2 + 2 + 1 = 5 electrons → boron (Z = 5).

Assignment:
104 MJ/mol (height 2) → 1s², the core electrons, held most tightly.
6.84 MJ/mol (height 2) → 2s².
3.67 MJ/mol (height 1) → 2p¹.

Justification: the 1s electrons experience nearly the full +5 nuclear charge with essentially no shielding, so their binding energy is more than an order of magnitude larger. The 2s electron penetrates closer to the nucleus than the 2p electron and is therefore less shielded, which is why 2s binds more tightly than 2p even though both are in the n = 2 shell.

Full notes for topic 1.6 →