CasaXPS Element Library

 

Elemental identification and quantification using XPS/AES spectra rely on the maintenance on libraries of peak positions and relative sensitivity factors. The default CasaXPS element library is compiled using rough peak positions and Scofield cross-sections for aluminium and magnesium X-ray anodes to represent the corresponding relative sensitivity of the photoelectric peaks relative to the C 1s transition. While appropriate for some, these peak energies and relative sensitivity factors will not satisfy all and therefore the CasaXPS system offers mechanisms for user and sample specific element library creation. This section describes the means for modifying the element library in CasaXPS.

 

The Element Library File Format

 

The CasaXPS element library is an ASCII file. The first line in the file is a version number (Figure 1), which may be ‘0’ or ‘1’ depending on whether the entries for a peak appear on separate lines or are separated by TAB characters, respectively. The latter, TAB spaced format is displayed in Figure 1 and supersedes the former with a view to improve the edit ability of the data in spreadsheet programs. For large number of changes, using a spreadsheet is the best means of constructing a CasaXPS library, however for small alterations the Element Library property page offers dialog window based adjustments of the library.

 

Figure 1: The CasaXPS default element library loaded into a spreadsheet.

 

 

Each peak within an element library entry consists of the following fields:

  1. Element
  2. Transition
  3. Label/Name
  4. Mass (Daltons)
  5. Energy Type (BE or KE)
  6. Energy (eV)
  7. F.W.H.M.
  8. Line shape (e.g. GL(30))
  9. Relative Sensitivity Factor
  10. Excitation source string

Figure 1 illustrates a set of peak entries in version 1 format. Note how the library entries visible include two different excitation source strings, namely “Al” and “Mg”. A single library file may contain any number of entries corresponding to different excitation source stings, however when used in CasaXPS, these strings are matched to the excitation source string in the displayed VAMAS block in the Active Tile and only those library entries for which a match occurs are displayed in the Element Table property page on the Element Library dialog window.

 

Although the version 1 library format is aimed at the creation of library files within a spreadsheet program, there exits a mechanism within CasaXPS for making occasional changes and additions to the currently loaded library file. Figure 2 shows the dialog window aimed at making these individual changes to an entry. The Edit Entry dialog window is invoked by placing the cursor over the Name field for an entry in the Element Table scrolled-list, then right-clicking the mouse button. The current set of library fields for the selected peak are displayed on the dialog window, from which these field can be adjusted, then either updated or a new entry created. To update the current entry, the OK button should be pressed, however a new entry is created, with the values taken from the state of the dialog window, when the Create button is pressed. Alternatively, the Delete button will remove the entry used to invoke the dialog window. Modifications to the element library, on exit, cause CasaXPS to offer to save the element library to a new file name.

 

Figure 2: Edit Entry dialog window used to make changes to the fields for an element library entry.

 

Since the default CasaXPS element library does not contain any relative sensitivity factors for AES peaks, the greatest need to create an element library is, therefore, for those using Auger data. The concepts and procedure are identical for XPS entries; however there are Auger specific features within the library mechanism and so AES spectra deserve special attention in the context of the element library. The Auger spectrum in Figure 2 will be used to illustrate editing the element library.

 

The Element Library and Auger Spectra: a General Example

 

When CasaXPS is started, the executable file CasaXPS.exe reads the library file CasaXPS.lib located in the same directory as the executable file (the name of the executable file determines the name for the default library file). When a spectrum is displayed in the Active Tile, on invoking the Element library dialog window, the Element Table property page displays those entries for which the excitation source string from the entry matches the corresponding field in the VAMAS block holding the spectrum. The exception to this rule occurs for AES data. If the excitation source field in the VAMAS block is set to ElectronGunDummy, then for AES spectra the electron gun energy is used to construct the string used to match an excitation source in the element library. That is to say, for an AES element library, the excitation source and relative sensitivity factors for electron beam energies 3 keV, 5 keV and 10 keV should be entered using the strings S(3), S(5) and S(10) (Figure 3). Sensitivity factors for AES are published in the book edited by Briggs and Grant (ISBN 1 901019 047).

 

Figure 3: Auger element library prepared for Cu and Ag transitions at electron gun energy of 5 keV.

 

Once an element library file is prepared for a particular application, the user may choose to make the library the default CasaXPS library or only load the library when appropriate. To make a CasaXPS formatted library file the default element library, the library file must be placed in the same directory as the CasaXPS executable and given the same base-name as the executable file. It is probably best to retain the standard naming conversion, namely, CasaXPS.exe for the executable and therefore CasaXPS.lib for the default library. Other configuration files and directories also use the base-name of the executable and therefore retaining the same base-name for all is important. Allowing the user to have several copies of the executable with different base-names permits custom behaviour. For example, it would be possible to set-up a copy of CasaXPS with the name CasaXPS_aes.exe, for which the library and configuration files would be named CasaXPS_aes.lib and CasaXPS_aes.DEF. These associated files are then free to be configured for use with Auger data. The alternative is to load a library file as and when required. To load a CasaXPS formatted library file, select the Input File property page (Figure 4) on the Element Library dialog window, use the Browse button to select the required library file, select the CasaXPS lib radio button and choose to either overwrite the existing library (Load button) or merge the new entries from the selected file with the current library (Merge button). Once a new file is either loaded or merged into the CasaXPS library, the entries can be examined via the Element Table property page and will be used when new quantification regions are created.

 

Figure 4: The Input File property page on the Element Library dialog window, where the derivative of the spectrum shown in Figure 2 is displayed using the element markers and relative sensitivity factors from the library in Figure 3.

 

The CasaXPS window in Figure 5 shows the differentiated Auger spectrum from Figure 2, where quantification regions are defined for each of the resonance structures from the Cu and Ag elements. Note that there are three regions defined for each of the Cu Auger lines and while regions for all three Cu lines are displayed on the spectrum, only the Cu1 peak has been assigned a non-zero relative sensitivity factor. Any quantification item, region or component, with zero RSF will be excluded from the resulting quantification table (also shown in Figure 5). Similarly, only one of the Ag peaks is assigned a non-zero RSF.

 

The quantification report in Figure 5 is generated using a configuration file defining the fields for inclusion in the table. The default Standard Report tabulates many fields not appropriate for AES data and therefore the configuration file RegionQuantTable.txt (located in the directory CasaXPS.DEF) is modified to extract only those values of interest for Auger data. The entries in the configuration file used to create the quantification report (Report Spec property page on the Quantification Parameters dialog window) are also included in the Notepad window shown in Figure 5. Once a report is created, the Copy toolbar button invokes a Copy to Clipboard dialog window, from which the table can be transferred via the clipboard to a spreadsheet or saved to a text file.

 

 Chemical state information can be extracted using quantification regions appropriately positioned to differentiate between shifted peaks. Figure 5 illustrates two closely positioned Ag peaks, which are plotted within the inset tile, where the region limits are defined to permit a measurement of the peak-to-peak intensity from each of the transitions. While these two peaks are not due to chemical state, for chemically shifted peaks, a similar judicious positioning of regions offers a means of measuring chemical information.

 

 

Figure 5: Differentiated Auger spectrum from an Ag Cu Alloy, where quantification regions are defined for every peak in the library file.