Electron inelastic mean free path in carbon and polycarbonate using a newly developed wide spectrum measurement method

Abstract

The electron inelastic mean free path (IMFP) is commonly used in surface analysis techniques such as X-ray photoelectron spectroscopy. Though numerous studies have been conducted in the quest to define the IMFP value, it is still substantiated mainly on the basis of calculations and extrapolations. Experimental methods such as elastic peak electron spectroscopy tend to rely on indirect measurements. The main disadvantage of current methods is that the signal to noise ratio decreases with the increase in layer thickness or as the electron energy decreases.The newly developed wide energy spectrum (WES) method previously published by our group allows the IMFP to be directly evaluated for different electron energies. It exploits the attenuated electron multiple inelastic collisions (MIC) peak counts to calculate mean electron energy loss, rather than use the signal-electron peaks.In contrast, currently accepted methods focus on the signal electron in the spectrum and disregard the information contained in the lower-energy electrons. In understanding how to exploit this information, WES enhanced the relevant data collection by using a wide energy region upper bounded by the signal electron energy and lower bounded by the peak MIC energy in the spectrum.In this work, we have markedly expanded the validity of the WES method by assessing the IMFP values of a low Z polymer and by using a different substrate to produce the photoelectrons. The experiment was performed at the Spanish CRG Beamline (SpLine) at the European Synchrotron Radiation Facility (ESRF). Herein we present our initial results. In addition, we improved both the raw-data analysis and the background reduction processes used in WES to obtain enhanced and more accurate results.The IMFP values of carbon and polycarbonate were evaluated and compared with those predicted by the NIST. Small differences notwithstanding, the reported IMFP were shown to be well within the uncertainty boundaries.This work shows that the electron IMFP can be evaluated in solids by directly measuring the electrons that pass-through samples of the solids of interest. For the first time, there is an orderly path to experimentally evaluate electron IMFP by using the WES method. The new method was established to address the lack of experimental IMFP values for low energy electrons in solid materials.