---
title: "XPS: Elements, Chemical States & Charging Interactive Lab | Sandbox Physics"
description: "How can you tell whether a shifted XPS peak comes from a chemical change or from charging? First reference the energy scale independently."
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page_type: "lab"
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  zh-CN: "https://sandboxphysics.com/zh/labs/xps-chemical-state/"
---

E51 · Photoelectron spectroscopy

# XPS: Elements, Chemical States & Charging

**How can you tell whether a shifted XPS peak comes from a chemical change or from charging?** First reference the energy scale independently. Charging moves every peak together; a chemical change moves one state relative to the others. After referencing, a constrained peak fit can back up a chemical interpretation.

Shine X-rays on an oxide layer on silicon, record a survey and two core-level regions, then fit the counts. Separate the chemical states, test how the energy scale is corrected for charging, and compare raw peak ratios with the surface composition after sensitivity correction.

> Interactive simulation: open this page in a web browser to change the parameters and run the model.

Model assumptions

The Si oxide on Si sample is synthetic. Both layers are assumed to have the same Si number density and a common 2 nm escape length, with ideal exponential escape and no elastic scattering. The lines sit at 99.4 and 103.5 eV for Si and 532.9 eV for O. The two Gaussian Si doublets have a 0.63 eV splitting and a 2:1 area ratio. Intrinsic widths, the continuum, the count scale and the relative sensitivities of 1 and 2.9 are teaching assumptions, not an instrument calibration. The Al and Mg source presets change the kinetic energy, while the assumed sensitivities and escape lengths stay fixed. A uniform positive potential on the sample raises every apparent binding energy, and neutralization sets how much of that potential remains. Weighted, constrained fits estimate the peak areas and a common shift from the recorded counts. The background is a smooth linear or quadratic continuum, not Shirley or Tougaard. You supply the reference correction. Here the Si anchor line can serve as a known substrate reference, but it is never a universal charge reference. The reported oxygen percentage is a surface-weighted, two-element equivalent; it is not bulk stoichiometry or an inverted depth profile. The survey and the high-resolution scans are recorded separately and have independent counting noise. Each exposure is taken as the integration time for one scan; animation and processing add no exposure. Noise is Poisson below 40 expected counts and a rounded Gaussian approximation above. The apparatus shows a fixed window of raw energies; fitting windows and reference corrections affect only the analysis. The analyzer width is a Gaussian equivalent for a 150 mm mean radius and a 2-degree entrance half-angle. Differential charging, satellites, Auger peaks, damage, real cross sections, exact electron optics and general chemical identification are left out.

Physics tutorial

## Chemistry needs a referenced energy scale

**Background** X-rays eject core electrons. Their kinetic energy tells you the element and its chemical surroundings, but the potentials of the spectrometer and the sample also shift the measured energy scale.

**Why it matters** Charging shifts every peak by the same amount, and that common shift can look like a chemical shift. Raw peak heights also mix together composition, sensitivity, escape depth, line width and counting noise.

Start with the essentials

- **Focus question**: Does a shifted peak mean a chemical change or charging?
- **One-sentence intuition**: Fit the real counts with stated line and background constraints, reference the energy scale independently, then integrate the peaks and apply consistent sensitivity factors. A layered surface still needs a depth model.

### Core mathematical model

#### Energy relation for a grounded sample, with residual charging

$$
E_{B,\mathrm{raw}}=h\nu-\phi_a-K_a=E_B+eV_c
$$

The analyzer work function sets the instrument's vacuum offset. A positive residual potential on the sample lowers the kinetic energy of arriving electrons, which raises the apparent binding energy; the reference correction subtracts a shift you supply.

#### How signal weight falls with depth

$$
w(z,\theta)=\exp\!\left[-\frac{z}{\lambda\cos\theta}\right]
$$

The angle is measured from the outward surface normal. The common 2 nm escape length is an assumption with no elastic scattering; it is not a universal effective attenuation length.

#### Share of the Si signal from the oxide

$$
f_{\mathrm{ox}}=1-\exp\!\left[-\frac{t}{\lambda\cos\theta}\right]
$$

Because both layers have the same Si density and escape length, the total Si signal does not depend on the oxide thickness. Grazing emission gives the oxide more weight but lowers the total escape integral.

#### Fit model: constrained doublets on a smooth background

$$
D_i\simeq A_0P_{0i}+A_{\mathrm{ox}}P_{\mathrm{ox},i}+b_0+b_1u_i+b_2u_i^2
$$

Each normalized Si template is a doublet with a 2:1 area ratio and a 0.63 eV separation. The areas are nonnegative, and a common energy shift and the background coefficients are fitted. These constraints are assumptions, and the fit is not a unique chemical diagnosis.

#### Two-element equivalent composition

$$
c_O=\frac{A_O/S_O}{A_O/S_O+A_{\mathrm{Si}}/S_{\mathrm{Si}}}
$$

The assumed sensitivities are 2.9 for O and 1 for Si. They show how the correction works and are not measured instrument factors. For a layered sample the result is a surface-weighted equivalent, not the atomic fraction of a uniform bulk.

### Common difficulties

#### Peak height is not peak area

**Typical misconception** The tallest peak belongs to the most abundant atoms.

**Better mental model** Widths, sensitivities and background all change peak height. Integrate the fitted counts and state the assumptions behind the quantification.

#### A reference value has limits

**Typical misconception** A standard carbon or Si value corrects any unknown sample.

**Better mental model** This Lab gives you a known substrate. Real charge referencing needs a justified reference, and it can fail when parts of the sample charge differently.

#### Fitting two components does not mean they were resolved

**Typical misconception** A constrained doublet fit proves both components were resolved.

**Better mental model** The model can return areas even when the instrument width merges the components. Check the width and the counts before you interpret the fit.

### Run the experiment

1. 01

   #### Predict a shift of every line

   Choose Chemistry or charging? and note the Si and O positions before you change the neutralization.

   **What to observe:** Every line moves together with the residual potential, and the gap between Si and its oxide stays the same.

2. 02

   #### Fit, then correct the energy scale

   Lower the pass energy, collect enough counts, turn on Fit a curved continuum and set the Processing reference correction from the known Si substrate line.

   **What to observe:** The fitted common shift comes from the counts. Changing the correction you supply moves only the energy scale on display.

3. 03

   #### Correct the area ratio

   Choose Reference and quantify, then switch Apply assumed sensitivity factors on and off.

   **What to observe:** The composition changes, while the recorded spectra and fitted areas stay the same.

4. 04

   #### Test surface sensitivity

   Compare normal and grazing emission at the same known oxide thickness, then complete the target.

   **What to observe:** The oxide's share of the signal grows. The reported oxygen fraction changes because of depth weighting; no oxygen has been created.

Physical conclusion

A properly referenced peak can support a chemical interpretation within the fit's constraints. A sensitivity-corrected percentage does not by itself give the bulk stoichiometry.

## Related experiments and articles

Electron probes · Reading path

### Electron probes: reading the evidence

Articles on this path that use this experiment, followed by the other experiments they suggest.

1. [Article **Kai Siegbahn: when electron energies began to reveal chemistry**](https://sandboxphysics.com/stories/electron-evidence/siegbahn-electron-spectroscopy-chemical-shifts/)
2. [Lab **Photoemission: Retarding Current & Energy Analysis** Separate emission from energy selection](https://sandboxphysics.com/labs/photoemission-analyzer/)

Fields & matter · Starter path

## From a field to an electron microscope

Map an electric field and steer a charged particle. Then find the electron's wavelength and use electron beams to see a crystal and a surface.

[Start here → **Make an Electric Field Visible** Move the probe around a dipole and check that the field always points downhill on the potential landscape. Observe · 8 min](https://sandboxphysics.com/labs/electric-field/)

1. [1 Make an Electric Field Visible](https://sandboxphysics.com/labs/electric-field/)
2. [2 A Charge Bending through Fields](https://sandboxphysics.com/labs/charged-particle/)
3. [3 Electron Wavelength & Voltage](https://sandboxphysics.com/labs/electron-wavelength/)
4. [4 Electron Diffraction & Reciprocal Space](https://sandboxphysics.com/labs/electron-diffraction/)
5. [5 TEM Bright Field, Dark Field & SAED](https://sandboxphysics.com/labs/tem-mode-selector/)
6. [6 SEM Secondary-Electron Topography](https://sandboxphysics.com/labs/sem-topography/)

[View the full path →](https://sandboxphysics.com/topics/fields-and-matter/#starter-path)