Method to correct ambient pressure XPS for the distortion caused by the gas

Sven Tougaarda and Mark Greinerb
aDepartment of Physics, Chemistry and Pharmacy, University of Southern Denmark, Odense, Denmark
bMax-Planck Institute for Chemical Energy Conversion, Department of Heterogeneous Reactions / Surface Analysis Group, Mülheim an der Ruhr, Germany

Applied Surface Science

Volume 530, 15 November 2020, 147243

With the development of beamlines such as ISSIS at Bessy and B07 at Diamond, along with a multitude of lab-based systems (such as the HarwellXPS partner facility in Manchester), near-ambient pressure XPS (NAP-XPS) has undergone a boom in interest and publication in recent years. One thing, however, has plagued data interpretation during this period. Scattering effects caused by the gas may alter the energies of the emitted photoelectrons, complicating the resultant spectra (despite the distance between sample and inlet to the differential pumping station typically being less than 1 IMFP length).

NAP-XPS – Walton research group, University of Manchester
NAP-XPS system at the University of Manchester (HarwellXPS affiliate)

Sven Tougaard and Mark Grenier have investigated this distortion and developed a method for the correction of the data. The approach they use is similar to that of that of using the inelastic background to determine surface nano-structure by XPS and uses the QUASES-Tougaard software developed by Sven Tougaard. This software contains a tool which may remove backgrounds caused by inelastic scattering.

The parameters which were deemed paramount are the IMFP (λ) and the inelastic scattering cross section, K(T) where K(T)dT is the probability of an inelastic scattering event during T,T + dT per unit distance travelled.

A spectrum of clean gold foil is taken under vacuum and under 5mbar of N2 and these, along with the background removal tool within the QUASES software are used to determine a value for the scattering cross section. The validity of these parameters are then cross checked against a number of spectra taken at different N2 pressures (at which point effective gas path length becomes a variable).

Once these values have been determined for this gas system, they may be applied to non-model samples other than pure gold foil.

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