Inelastic mean free path (IMFP – λ) defined as the distance travelled by an electron of a given energy between successive collisions

IMFP dependant on material properties (e.g. density, number of valence electrons, band gap)

This process gives XPS it’s surface sensitivity

Following a photoemission process, electrons may undergo inelastic scattering collisions

Inelastic collisions remove kinetic energy from the photoelectron

Spectrometer sees this as a higher effective binding energy

Probability of undergoing an inelastic collision is proportional to the depth at which the photoelectron originated

Roughly one sigma (σ), which is 68%, of all photoelectrons able to reach the vacuum originate within the first λ nm deep

The information depth in XPS is typically defined as 3λ, where this becomes 99.7% of all photoelectrons

Backgrounds are produced by the inelastic scattering of released photoelectrons

Every photoemission peak will contribute to the background

At high binding energies the background is the contribution from all core levels of all elements!

Eventually, we will likely want to be able to develop some fairly complex peak fitting models

But even to develop simple ones, we need to accurately describe the background before we can describe the resulting peak

XPS analysis software (such as CasaXPS) provides a simple method by which to apply dynamic backgrounds to our spectra to model the scattering processes

Most straightforward background

Straight line from A to B

Can be used for simple systems (e.g. polymers) but in most cases linear backgrounds are not sufficient for data analysis

Shirley backgrounds are a background generated based on the proportional ratios of integrated peak area to the high and low binding energy side

Work perfectly for valence bands

Appropriate for vast majority of core spectra – generally recommended for beginners due to the ease of use

Shirley backgrounds reduce asymmetry in peaks

Appropriate lineshapes must be used (to be covered later in this course)

Tougaard backgrounds the most theoretically robust treatment of an XPS spectrum

Used in a number of advanced analytical techniques

Will not be covered in this beginners course