XPS for Beginners

Fundamentals 1

Upon exiting the atom, a photoelectron may promote the resulting ion into a excited state

This process costs the photoelectron an amount of kinetic energy, meaning it will be recorded as if it possessed a higher binding energy within the atom

The peaks produced by this method are called ‘Shake-up satellites’

For transition metals, such as Cu, the shake-up can be described as a strong interaction of the final states

This involves charge transfer from the O 2p band to the Cu 3d band

This effectively results in a final configuration of 2p53d9

Metals and conductive materials may exhibit a special type of shake-up process caused by the conduction band

The continuum of energies produces photoelectrons which lose a corresponding continuum of kinetic energy

Results in peaks which exhibit extended tails on the high binding energy side

Degree of asymmetry depends heavily on DOS near Fermi level

High DOS = more of a continuum for shake-ups

E.g. Au and Pt

Another good example is first row TMs

As we fill the d-band we lose asymmetry

Materials with high density of free electrons around the Fermi level may form plasmon peaks

Plasmons are oscillations in the ‘sea’ of electrons around the Fermi level

Photoelectrons may interact with this plasmon upon exiting the material and lose a defined amount of energy

Interval between plasmons constant

Sometimes plasmonic peaks arising from surface plasmons and bulk plasmons may be separated

Al, Mg, Si, Na typically exhibit strong plasmon structure

Following photoemission, the atom will respond to the newly created core hole

This response results in so-called ‘final state effects’

E.g. Lanthanum – produces 2 sets of doublets by XPS

2 peaks near 835 eV = 3d5/2

2 peaks near 850 = 3d3/2


Higher binding energy peaks = unshielded core emission

Lower binding energy peaks = shielded core emission

Following photoemission, Lanthanum may rearrange to reduce the total energy of the system via charge transfer from the conduction band to the 4f orbitals forming the configuration:  4fn+1(3dn+1) – cf1L

Alternatively, the system may stay in the ground state: 4fn(3dn) – cf0 – resulting in a higher binding energy peak

Core hole notation: c = La core hole, L = ligand core hole

Unpaired electrons may cause multiplet structure in XPS spectra

Coupling with the newly created core-hole creates a number of final-states resulting in multiple emission peaks

First-row transition metals especially affected

This complicates data analysis greatly

E.g. Ni free ion envelopes (i.e. no final-state effects from ligands)

Many peaks arise due to unpaired electron coupling

These multiplet fits combined with shake-up satellites and other final state effects form the complex structures seen in first row TM compounds

It is often advised to analyse these spectra qualitatively where possible

Procedures for chemical deconvolution exist – but are hard!