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Why XPS can't detects Hydrogen & Helium

Why XPS can’t detect hydrogen (H) and Helium (He)? X-rays Photoelectrons Spectroscopy (XPS) technique analyze All elements Except H & He. The following are the three main reasons. 1. XPS works with core electrons NOT with valence electrons. Here, H and He have only valence shell electrons. Core electrons are important because they are not affected by the surrounding atoms/elements. Therefore, they are carrying the accurate information of the elements. 2. The cross section is very low for XPS in case of valence shell electrons, thereby very less probability for x-rays photons and electrons interaction. If we look into the valence shell electrons, there is more spaces as compared to the core shell electrons. Therefore, the probability of interaction between x-rays photon and Valence shell electrons are LESS and vice versa. 3. The core electrons respond very well to the x-rays photons due to the fact that MOST of the elements have Binding Energy (BE) in the range of ~1200 eV whereas the XPS x-rays sources are Al ~1486.6 eV and Mg ~ 1253.6 are in the same range. These x-rays energy sources are designed for core electrons only. Please do comments if you disagree or if you have additional information! 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    • Fundamental Equation "hv = BE + KE + ...   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)?    • What is Binding Energy in X-rays Phot...   Importance of Survey Spectra in XPS - X rays Photoelectron Spectroscopy    • Survey Spectra in  X-rays Photoelectr...   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 -...