Teignmouth Science and Technology Centre (TSTC)
providing access to science, mathematics and technology
Copyright © 2017 Teignmouth Science and Technology Centre (TSTC)
Click on the line below for free pdfs
Provided with the express permission of Casa Software Ltd. (known as CasaXPS)
1. Introduction to surface analysis using XPS (X-Ray Photoelectron Spectroscopy)
An x-ray photoelectron spectrum is a sequence of measurements, where each measurement records the numbers electrons of a given energy for a specific time interval. The objective is to measure counts per second (CPS) as a function of kinetic energy (eV) of emission for electrons from atoms in the surface material. Measured in units of CPSeV, variation in intensity for electrons with differing emission energies provides a means for characterising the electronic state of atoms within the sample from which electrons are emitted and offers quantifiable information about the surface composition.
An iXPS data set consists of a spectrum at every pixel in an image. These spectroscopic image data sets may exhibit lateral differential charging as a consequence of:
· An uneven X-ray flux
· Different intensity of photoelectron emission across the field of view
This is not necessarily a problem for quantification where the limits of integration can be adjusted to accommodate the charging. Neither is it a problem for peak fitting where peaks can be fixed with respect to the primary peak which can be allowed to move within limits. However, it is a problem for Principal Component Analysis (PCA) which is used to improve the signal/noise in the iXPS data set. PCA recognises different charge shifts as a separate component increasing the number of components and therefore the possibility that some will be lost in the noise of the data. Correction of charging at every pixel is therefore necessary. This page illustrates the steps required to do this.