X-Ray Photoelectron spectroscopy

lonization Energy

Energy required to remove an electron to an infinite distance at rest from an atom, molecule, or ion.

Photoelectric Effect

Observation that a minimum frequency (energy) of light was needed to remove any electrons (photemmision), minimum energy corresponds to the work function o of the material.

Metal energy diagram

Photoemmision Event

Detection Event

Energy level alignment

• Not connected: KE₀ levels aligned (same vacuum levels.

• Connected: E_F levels are aligned.

KE work function relationship

XPS requirements

• Sample and detector at same potential

• accurate value of energy source

Principles: Fermi distribution (T=0K)

Cause: At temperatures T ≠ 0, the fermi level is smeared out according to the fermi distribution function.

Effect: Smearing of valence band

Principles: Uncertainty principle

ΔΕ Δt = h/4π

• ΔΕ: energy broadening

• Δt: life-time of electron

Principles: Electron Spin

Electrons have spin (± 1/2), diffrent spin orbit coupling means diffrent binding energies.

• j-j: momenta Z >75

• l ± s: sum of total spin and orbit angular momenta - proportinal to ratio of occupancy relative

Principles: Intensity

Efficiency of phton interaction with the electron is dependent on phtoelectron cross section.

Low cross section low intensity.

Electron Distribution Curve

Inelastic Scattering: loss peaks

when the primary electron looses energy due to a single scattering event as it leaves the sample

Inelastic Scattering: secondary electrons

When the primary electron scatters multiple times and causes other (low energy) electrons to be ejected from the material

Principles: Inelastic scattering

Causes an increase in the background level on the high binding energy side of all peaks in an XPS spectrum.

Auger scattering

Create a hole in core level - electron falling ejects auger electron

Principles: X-ray satellites

X-ray sources have finite line widths.

secondary emmision line at lower binding energy to main peak.

Analyzer transmission

Acceptance aperature is finite - line broadening.

Kinetic energy of transmitted electron not all equal - discrepencies in peak heights

Peak Positions

Info:

• elements in the material

• oxidation state

Error factors:

• overlapping peaks

• charging

• satelites

Peak half widths

Info:

• photoemission llifetime

• oxidation state

Error factors:

• overlapping peaks

• source/analyzer broadening

Peak areas

Info:

• concentration

Error factors:

• overlapping peaks

• sampling depth

Chemical State Effects

factors which influence the state charge of an atom before a photon strikes it.

Hybridization

The BE of an electron in agiven orbital changes due to a change in hybrization state of the atom.

Oxidation State

The BE of an electron increases as the oxidation state of the atom increases. Tracks with 1/R

Degree of Bonding

As the atom looses valence electrons, the BE of all remaining electrons increases. Electronegativity/atom sizes/no of neighbouring bonds determine magnitude

Chemical state effects: Magnitude

When an electronic change occurs in an atom, the change in potential felt by an electron in an orbital at R from nucleus goes 1/R.

• more significant for core orbitals (small R)

• decrease going down the group (increase in R)

• Increase across period (decrease in R)

Angle-Dependent XPS

Angle of electron emission is varied, improving depth profiling without destructive sputtering.

Escape Depth and Inelastic Mean Free Path

Electrons emitted from a sample undergo inelastic scattering, limiting their escape depth. The IMFP (λ) is the average distance an electron travels before losing energy.

Angle Dependence

effective escape depth (d) of photoelectrons

Signal Intensity

Detected photoelectron signal diminishes exponentially with depth - analogous to beer lambert law