Version 2.2 of SESSA can be used to simulate AES and XPS spectra of nanostructures such as islands, lines, spheres, and layered spheres on surfaces. As for earlier versions, such simulations can be performed for multilayer films. Users can specify the compositions and dimensions of each material in the sample structure as well as the measurement configuration, and the simulated spectra can be compared with measured spectra. Compositions and dimensions can then be adjusted to find maximum consistency between simulated and measured spectra.
This database has been designed to facilitate quantitative interpretation of AES and XPS spectra and to improve the accuracy of quantitation in routine analysis [1-3]. SESSA contains physical data needed to perform quantitative interpretation of an AES or XPS spectrum for a specimen of given composition and morphology (differential inverse inelastic mean free paths, total inelastic mean free paths, differential elastic-scattering cross sections, total elastic-scattering cross sections, transport cross sections, photoionization cross sections, photoionization asymmetry parameters, electron-impact ionization cross sections, photoelectron lineshapes, Auger-electron lineshapes, fluorescence yields, and Auger-electron backscattering factors). Retrieval of relevant data is performed by a small expert system that queries the comprehensive databases. A simulation module provides an estimate of peak intensities as well as the peak spectra.
The design of the software allows the user to enter the required information in a reasonably simple way. The modular structure of the user interface closely matches that of the usual control units on a real instrument. Any user who is familiar with a typical AES or XPS spectrometer can perform a retrieval/simulation operation with the SESSA software in a few minutes for a specimen with a given composition and morphology. A command line interface can also control the software; this feature allows users to load sequences of commands that facilitate simulations for similar conditions.
Version 1.0 of this database was released in December 2005. Version 1.1 was released in December 2006 with an enhancement to the Model Calculation screen that permits the user to display and save the zero-order partial intensities. Previously, a user had to go to another screen to perform these operations. Version 1.2 was released in March 2010 with the following enhancements: an additional and more intuitive format for specifying the composition of a material; a new capability to perform simulations with polarized photons; the ability to save plots in additional file formats; the addition of a chemical-shift database for selected peaks; improvements in the peak-management software; and incorporation of a faster random number generator. In addition, an internet SESSA forum has been established for user questions and a new SESSA bug-tracking web page has been established. Version 1.3 was released in May 2011 with a new database of non-dipole photoionization cross sections that are necessary in simulations of X-ray photoelectron intensities with X-ray energies higher than a few keV. In addition, a description is given of how SESSA can be called and controlled from an external application. Version 2.0 was released in October 2014 with additional capabilities for specifying specimen nanomorphologies (such as islands, lines, spheres, and layered spheres on surfaces) and with updated data for electron inelastic mean free paths. Version 2.1 was released in December 2017 with corrections of two software bugs in SESSA 2.0 that affected simulations with the islands morphology and simulations with the spheres morphology if one or more additional layers had been added to the substrate. This version allows users to create new sample morphologies with the PENGEOM geometry package  and has two new databases, one for the inelastic mean free path and another for the electron-impact ionization cross section. Version 2.1.1 was released in August 2018 to correct a software bug that affected calculations of photoelectron intensities involving non-dipole terms in the photoionization cross section. This bug could have significant effects only for X-ray energies in excess of 2 keV. Version 2.2 was released in March 2021. It contains a new database for inelastic mean free paths. Surface excitations have been added to the simulations as well as an effective approximation to account for the energy dependence of the inelastic mean free path. A Quit command has been implemented which allows users to terminate the program using the command line interpreter, either at the terminal or in a session file. While this option is very useful for making possible a certain kind of batch operation, care should be taken with session files written for older versions of the software as an unintended quit command in any session file will terminate the program.
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Important Note to Users of the Mac Version of SESSA
The Apple Macintosh version of the SESSA Version 2.2 software operates in unexpected ways at times. This problem is caused by a bug in the tools used to create the graphical user interface. Any numerical value displayed in SESSA can be changed manually by a user. Alternatively, by means of a right mouse click in the box in which the value appears, one can activate a popup menu. This menu can be used to either select the database or inspect the reference in the scientific literature from which the value was retrieved, as described in the manual. Unfortunately, instead of the regular behavior, a general-purpose menu pops up at times that is not meaningful within the context of the SESSA software. The remedy for a workaround is for a user to perform a right click in another box with a numerical value (e.g., one of the neighboring boxes) and make another attempt in the desired box. It may be necessary to repeat this process an undefined number of times (typically one to three). We regret this inconvenience. However, the problem originates from within the software tools used by the SESSA developers to create the graphical user interface, and it is presently not possible to solve this problem. It is hoped that the problem will disappear in a natural way in future versions after it is resolved in the software tool.
Finally, it should be emphasized that the functionality of the command line interface is not at all affected. In addition, users who do not need the new features of SESSA Version 2.2 can download Version 2.1.1 where this bug is absent.
SESSA runs on personal computers using the Windows operating system including Windows 7 32-bit and 64-bit systems. The databases and software require a hard disc space of approximately 180 MB. The minimum amount of RAM needed to run SESSA is about 15 MB, but 30 MB or more is needed for simulations. SESSA is also available for MacIntosh OS X and Linux operating systems, but these versions have not been as extensively tested as the Windows version, and are not supported.
For more information please contact:
Standard Reference Data Program
National Institute of Standards and Technology
100 Bureau Dr., Stop 6410
Gaithersburg, MD 20899-6410
(844) 374-0183 (Toll Free) email@example.com (E-MAIL)
The scientific contact for the database is:
National Institute of Standard and Technology
Materials Measurement Science Division (643)
phone: (301) 975-2534
1. W. Smekal, W. S. M. Werner, and C. J. Powell, Surf. Interface Anal. 37, 1059 (2005).
2. W. S. M. Werner, Surf. Interface Anal. 31, 141 (2001).
3. W. S. M. Werner, in Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, D. Briggs and J. T. Grant, eds. (IMPublications, Chichester, 2003), p. 235.
4. J. Almansa, F. Salvat-Pujol, G. Diaz-Londono, A. Carnicer, A. M. Lallena, and F. Salvat, Comput. Phys. Comm. 199, 102 (2016).
Keywords: Auger-electron backscattering factors, Auger-electron lineshapes, Auger electron spectroscopy, cross sections, elastic scattering, electron-impact ionization cross section, electron scattering, electron transport, fluorescence yields, inelastic mean free paths, inelastic scattering, photoelectron lineshapes, photoionization asymmetry parameters, photoionization cross sections, surface analysis, transport cross sections, x-ray photoelectron spectroscopy.