Multivariate Analysis for Spectromicroscopy : Getting More From XPS Imaging

The spherical mirror analyser (SMA) has been employed on a commercially available photoelectron spectrometer for ten years.  During this time numerous examples of both elemental and chemical state images have been published and x-ray photoelectron imaging has become a routine technique for the determination of lateral distribution of elements and chemical species at the surface. 

Here we review the properties of the SMA including fast parallel image acquisition, high spatial and energy resolution and provide examples of the capabilities of such an imaging analyser.  In the last three years the combination of the SMA with a two-dimensional, pulse counting electron detector has again increased the level of information available for surface characterisation.  The delay-line detector (DLD) represents the next generation of photoelectron detection for XPS imaging and has allowed the realisation of quantitative surface chemical state microscopy by x-ray photoelectron spectroscopy.  To generate such information requires the acquisition of multi-spectral datasets comprising a series of images incremented in energy so that each pixel contains photoelectron intensity as a function of energy.  The datasets generated by this method contain >65,500 spectra and are therefore ideally suited to multivariate analysis to analyse the information content of the dataset and as a tool for noise reduction in individual images or spectra.  Several examples including a plasma modified surface and corrosion of a medical grade steel are used to demonstrate parallel imaging using the SMA and the additional information that can be gained by using multivariate statistical analysis of multi-spectral datasets.