Attenuation lengths in organic materials

Measurements are reported for attenuation lengths in overlayers of guanine, poly(styrene), poly(methyl methacrylate) and poly(2-vinylpyridine) in the energy range 700-1400 eV to evaluate the accuracy of theoretical computations and generic equations. The layers are deposited on gold, either by evaporation and condensation or by spin casting. Measurements are made by X-ray photoelectron spectroscopy (XPS) for the substrate Au 4f, 4d and 4p peaks as well as the overlayer N 1s peak in guanine and poly(2-vinylpyridine). These measurements all correlate with the theory of Tanuma, Powell and Penn, to an RMS deviation of 11%. Correlations with the predictions using the generic equation known as TPP-2M exhibit a poorer RMS deviation of 20%. Use of an 'average' organic material for the inelastic mean free paths for the four materials also leads to a 20% RMS deviation. Additional data are also presented for Irganox 1010 that is used, increasingly, as a reference sample for secondary ion mass spectrometry. Comparisons for 5 materials with the Gries G1 formula gives an RMS deviation of 13%, but a small change to interpolate between Gries' classes of organic materials with H/C being around either 1 or 2, leads to a reduced RMS deviation of 10%. This final version, G1-SS, requiring only the material density and chemical formula, is very simple to use for all organic materials where these data are known or can be estimated. (c) Crown copyright 2010. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.