Elastic Peak Electron Spectroscopy Theorized for Surface Analysis

A theoretical model describing the sampling depth of elastic peak electron spectroscopy for calculating the parameters of electron scattering in the surface layers of samples may enable more reliable interpretation of measurement data and reduce the time needed for materials analysis. Traditionally, multiple calculations have been required to determine if the electrons reflected from the surface of the material absolutely pertained to the sample being tested, and not, for example, to the base on which the sample was located.

Professor Aleksander Jablonski from the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw, Poland, at the apparatus for surface spectroscopy. Courtesy of IPC PAS/Grzegorz Krzyzewski.

Professor Aleksander Jablonski of the Polish Academy of Sciences has reported a theory for calculating the parameters of electron scattering in the surface layers of samples in order to reduce the time required for these calculations. Jablonski’s model uses analytical formulas containing certain constants that, although they may take weeks to calculate, need to be calculated only once for each individual element used in the analysis.

"The problem looks like this: I have a sample, I shoot electrons at it, I record those that are emitted from the surface, and on that basis I try to say something about the material. But of all the electrons fired in the direction of the material only some of them will bounce back!” said Jablonski. “In addition, my detector does not pick all of them up, it responds only to those which hit it. So if I now want to interpret the experimental data by comparing them with the results of simulations of electron trajectories, I have to perform very, very many, sometimes tens of millions of these simulations.”

Electron backscattering plays a fundamental role in surface spectroscopy. Photon backscattering in thick and dense atmospheres of other planets enables us to admire the beauty of Venus, Jupiter or Saturn. Courtesy of IPC PAS.

Jablonski’s analytical model produced results for penetration depth distribution function, mean penetration depth (MPD) and information depth (ID) comparable to results achieved using Monte Carlo simulations. In addition, the model was able to achieve results much more quickly. In comparisons performed on four elements (AI, Ni, Pd and Au), the model practically reproduced complicated emission angle dependencies of the MPDs and the IDs, correctly indicating numerous maximum and minimum positions.

Jablonski also proposed a way to determine the inelastic mean free path (IMFP) from elastic peak electron spectroscopy and proposed criteria for a minimum overlayer thickness for the IMFP measurements. Many materials, because they are difficult to polish to a state of true flatness at the atomic level, are deposited on a smooth surface. This has raised the question of how thick the deposited layer should be, in order to ensure that it is the layer that is being studied and not the support.

Researchers now have a means to determine parameters for electron scattering more rapidly and accurately using Jablonski’s novel theory of electron backscattering.

The research was published in Applied Surface Science (doi:10.1016/j.apsusc.2016.03.176).