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Basis of XPS Xrays Photoelectrons Spectroscopy
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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)? • • Why XPS can't detects Hydrogen Helium • 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 • • MOST Important Equation of XPS • XPS vs XRF vs Auger Effect- X-rays Photoelectron Spectroscopy • • XPS vs XRF vs Auger Effect- X-rays Ph... • What is Binding Energy (BE) in X-rays Photoelectron Spectroscopy (XPS)? • • Binding Energy in XPS • Importance of Survey Spectra in XPS - X rays Photoelectron Spectroscopy • • What is Survey Spectra in XPS? • Why p-orbital, d-orbital, f-orbital have TWO Peaks- Doublet in XPS Spectra • • Why p-orbital, d-orbital, f-orbital h... • Why XPS is a Surface Sensitive Technique? • • Why X-rays Photoelectron Spectroscopy... • Why Only Core Electrons Peaks in XPS - X-rays Photoelectron Spectroscopy • • Why XPS Only Analyze Core Electrons -...
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