The interaction of X-rays with samples involves three primary phenomena: interactions with metals, atoms, and electrons. The focus will be on the third phenomenon related to X-ray Photoelectron Spectroscopy (XPS).
Limitations in Detecting Elements
XPS cannot detect hydrogen and helium because these elements do not interact with X-rays; the photons pass through without any interaction, resulting in no detectable signals.
Electron Interaction Dynamics
When X-ray photons interact with electrons, they scatter in different directions. This scattering results in energy loss for the photon, a process known as Compton scattering.
Basis of X-ray Photoelectron Spectroscopy (XPS)
The third phenomenon is crucial for understanding XPS: when an X-ray photon transfers all its energy to a core electron, ejecting it from the sample. This process is described by the photoelectric effect equation relating photon energy to binding energy and kinetic energy of the ejected electron.
X-ray interaction with matter / atoms or electrons - The basis of x-rays photoelectrons Spectroscopy (XPS)
When x-rays photon strike upon the atom, one of three events may occur:
1. The x-rays photon may traverse through the atom with no interaction- Nothing happen. This is why XPS can't detect Hydrogen and Helium.
2. The x-rays photon can be scattered by an atomic orbital electron leading to partial energy loss, which refer as ‘Compton scattering.
3. The x-rays photon can interact with an atomic orbital electron with total transfer of photon energy to the electron, leading to electron emission from the atom and this is the basis of XPS or ESCA
ESCA stands for Electron Spectroscopy for Chemical Analysis
XPS stands for X-rays photoelectron spectroscopy
Why XPS can’t detect hydrogen (H)and Helium (He)?
https://youtu.be/NKgbKZHcFjg
Due to the following three reasons. X-rays photons Spectroscopy (XPS) can't detect the H & He.
1. XPS works with core electrons. Where H and He have only valence shell.
2. The cross section is very low for XPS, thereby very less probability for x-rays photons and electrons interaction.
3. The core electrons respond very well to the x-rays photons
XPS is a surface analysis!
X-ray Photoelectron Spectroscopy (XPS) is a key and very important surface analysis technique. In this technique, an x-rays is simply bombarded on the sample, and the photoelectrons emitted from the core of the atom. The Kinetic Energy (KE) of the photoelectrons then detected by the XPS detector. Using the following equation, the Binding Energy (BE) can be calculated
BE = hv - KE - ϕspec
Where 'hv' is the x-rays energy (constant value), ϕspec is the work function of the spectrometer (a constant value). Therefore, the BE of the element can be calculated, which can reveal many important characteristics about the element. For instance, what element exists, what the atom or element bonded with, the chemical environment, oxidation states, whether the electrons removed or added to the atom.....
Question: Why is Studying Surface important?
Response: All materials contain surfaces, which interact with other materials. For instance, the key properties such as surface wettability, corrosion, adhesion, charge transfer, and CATALYSIS are all determined by surfaces, so surface analysis is important and XPS can be utilized to characterized the surfaces.
Question: What is Surface?
Response: A surface is an extremely thin layer, i.e., surface = 3 atomic layers (~1 nm). Size wise, the surface, ultra-thin film, and thin film are related by the following relation
*Surface* smaller than *Ultra-thin film* smaller than *Thin film*
Question: Why is XPS Surface Sensitive Technique?
Response: Approximately 95% of the photoelectrons are emitted from the ~10 nm depth surface. This is why XPS is termed as surface sensitive analysis. However, x-rays can penetrate to a few micro meter depths in the sample, but due to inelastic scattering and the smaller free mean paths of electrons, the photoelectron lost all of KE and can not reach the XPS detector
In XPS, x-rays completely transfer its energy to the core electron and there are Photogenerated electron, Auger electron, & XRF phenomena (this one is weaker and can not be considered). Therefore, the XPS spectrum contains only the photoelectron peaks and Auger electron peaks.
In the XPS spectrum, the number of electrons detected are plotted on the y-axis while the BE plotted on the x-axis.
Secret Behind "hv = BE+KE+Ø" Equation for X-rays Photoelectron Spectroscopy
https://youtu.be/zKbBA-Mdcqg
XPS vs XRF vs Auger Effect- X-rays Photoelectron Spectroscopy
https://youtu.be/WMSiReWsUCk
What is Binding Energy (BE) in X-rays Photoelectron Spectroscopy (XPS)?
https://youtu.be/gImrgl8Mp7k
Importance of Survey Spectra in XPS - X rays Photoelectron Spectroscopy
https://youtu.be/t4WJ3_sXbzY
Why p-orbital, d-orbital, f-orbital have TWO Peaks- Doublet in XPS Spectra
https://youtu.be/LL281sSvdDc
Why XPS is a Surface Sensitive Technique?
https://youtu.be/xb3jZ7Z9EoU
Why Only Core Electrons Peaks in XPS - X-rays Photoelectron Spectroscopy
https://youtu.be/lOGKcXS11RM
