Quick Overview #
If we want to understand the various chemical components of our system from our XPS data, we can use CasaXPS to peak model multiple environments and reveal the underlying surface chemistry.
CasaXPS contains many powerful options for peak modelling, and here we outline a quick start guide to learning to peak fit.
If we want to understand the various chemical components of our system from our XPS data, we can use CasaXPS to peak model multiple environments and reveal the underlying surface chemistry.
CasaXPS contains many powerful options for peak modelling, and here we outline a quick start guide to learning to peak fit.
Adding a Background #
Backgrounds are added when we add a new quantification region in CasaXPS. See our page on quickstart to quantification if you haven’t already, and want to learn more about this.
Use the quantify window (F7), and in the regions tab, click create. You can then drag and drop the sides of the region to fit around the peak.
Background Properties #
| Parameter | Meaning |
|---|---|
| Name | The name of the region, usually corresponding to the core level (e.g., C 1s for carbon 1s orbital). |
| R.S.F. | Relative Sensitivity Factor: Used to correct for differences in detection sensitivity across different elements. |
| Start | The beginning of the energy window (in eV) for integration or fitting. |
| End | The end of the energy window (in eV) for integration or fitting. |
| BG Type | Background Type: The method used to subtract the background (e.g., Shirley, Tougaard, Linear). Shirley is the most common method. |
| Av. Width | Averaging width, controls how many data points are used to determine the background value at start and end points. |
| St. Offset | Start Offset: Adjusts the starting position of the integration or fit. |
| End Offset | End Offset: Adjusts the ending position of the integration or fit. |
| Cross Section | Cross sections for Tougaard backgrounds, and as parameters for other advanced backgrounds (e.g. Quadratic, Bilinear etc. |
| Tag | Used for advanced reporting |
| Area | Integrated area under the peak after background subtraction (corresponds to the number of photoelectrons). |
| Std Dev Area | Standard deviation or uncertainty in the area measurement. Can be generated using the Calculate Error Bars function. |
| fwhm | Full Width at Half Maximum: Measures the peak width, indicating resolution or broadening. |
| Position | Peak position (binding energy, in eV). |
| % Concentr. | Percentage concentration of the element. |
| Max Height | Maximum height (intensity) of the peak. |
| Min Height | Minimum height (likely of the background or after background subtraction). |
| Peak to Peak %Conc | Peak-to-peak concentration change expressed as a percentage. |
Adding Peaks #
After a background has been created, you can use the components tab to start adding peaks. Peaks can be dragged around by clicking and dragging the top of the peak. Peak widths can be changed by clicking and dragging the side of the peak. It’s useful to use the peak constraints options to control the fit (see Peak Properties section), and avoid poor fitting. Peaks can be locked relative to another peak (e.g. using A*1 in colummn B FWHM constraint, to lock the peak widths to the same value), or set within a boundary (e.g., typing ‘ #0.1 ‘ and hitting enter, sets the constraints to + or – 0.1 of the current value).
Peak Modelling #
| Field | Meaning |
|---|---|
| Component | The label/name of the component (all are C 1s here, labeled A, B, and C). |
| Name | Again indicates the orbital (C 1s) — helpful if multiple components from different orbitals are present. |
| R.S.F. | Relative Sensitivity Factor — same for all C 1s components here. |
| Line Shape | Type of line used for peak fitting: GL(30) means a Gaussian-Lorentzian mix (30% Lorentzian). |
| Area | Integrated area under the component peak — proportional to the number of detected photoelectrons. |
| Area Constr. | Constraints applied to area during fitting. X, Y integers locks peak area to X < area < Y. This can be locked relative to another peak (e.g. Peak A) using A*X. When fitting doublets, use this to lock peak intensity ratios. |
| fwhm | Full Width at Half Maximum — peak width. A red value indicates that this parameter may be fixed or constrained. |
| fwhm Constr. | Allowed range for the fwhm during fitting (lower and upper bounds). This can be locked to another peak (e.g. A*1) |
| Position | Peak position (binding energy in eV). |
| Pos. Constr. | Constraints on peak position during fitting (e.g., must stay within 291.46–281.00 eV). This can be locked to another peak (e.g. A + 1.5) |
| Tag | Used for advanced reports and calculations |
| Comp Index | Index for internal tracking; -1 typically indicates no explicit constraint group. |
| Asymmetry Index | Used for asymmetric peak shapes (usually 0 for symmetric GL functions). |
| % Concentr. | Calculated relative concentration of this component within the region — these should sum to ~100%. |
Rules for XPS Fitting #
When fitting XPS data, it is important to keep in mind several important rules to ensure you do not draw erroneous conclusions from your data! (See our page on bad XPS in literature for more information!)
1. Use as few peaks as possible!
It is tempting to add more peaks and get a better fit, but can you really justify all of those peaks? Think about what could be in your sample, and try and minimise how many peaks you add.
2. Consistent FWHM
FWHM is directly linked to core-hole lifetime. For most systems, these should all be similar for the same element (except in the cases of metal vs oxide etc). Ensure your FWHM are not too different in your peak fits.
3. Spin-Orbit Coupling
Remember, your fits should obey the laws of physics!! If you are fitting anything other than an S orbital, you need to account for the doublet and ensure the peak area, FWHM ratios, and peak separation are correct.
4. Backgrounds
Backgrounds should go through the noise of the data well, and not intersect the spectra to appear above the raw data.
