Estimation of inelastic mean free paths for polymers and other organic materials: use of quantitative structure-property relationships

Inelastic mean free path values are needed for quantitative XPS analysis of a range of important polymers and other organic materials. Measured reference data are limited, although some semi-empirical equations to estimate IMFP are available. Existing estimation methods often require values for such quantities as the bandgap (in electron-volts) or the density, for which accurate measurements may not be available. Here we develop a scheme for estimation of the IMFP that can be applied across a range of organic structures for kinetic energies of similar to 200-2000 eV. This involves a quantitative structure-property relationship (QSPR), which allows the IMFP to be estimated for any organic material from its structural formula alone, either via a straightforward calculation on paper or instantly using one of a number of commercial molecular-modelling software packages. The accuracy of this IMFP estimate is better than existing methods, such as the TPP-2M equation for organic materials. In the majority of analytical problems involving surface analysis in chemistry or biochemistry, the structural formula of the organic material is known or can be conjectured with reasonable confidence, so that the approach described here is more widely applicable in practise and likely to be more accurate than previous methods for estimating the IMFP in organic materials. (C) Crown Copyright 2001 Reproduced by permission of the Controller of HMSO.