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XPS Simplified: Why Binding Energy Is Inversely Related to Kinetic Energy!
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XPS Simplified: Why Binding Energy Is Inversely Related to Kinetic Energy!

Understanding Binding Energy and Kinetic Energy in X-ray Photoelectron Spectroscopy

Key Concepts of Binding Energy

  • The binding energy of electrons is crucial in understanding their relationship with the nucleus; higher binding energy correlates with lower kinetic energy.
  • Core electrons, when exposed to X-rays, can escape from the sample as photoelectrons, which possess kinetic energy.

Electron Configuration and Binding Energy

  • In lead's electronic configuration, 4S electrons are closer to the nucleus than 4P, 4D, and 4F electrons, resulting in greater binding energy for those closer to the nucleus.
  • Core electrons (like those in the 3D subshell) exhibit high binding energies due to their proximity to the nucleus.

Relationship Between Photon Energy and Kinetic Energy

  • The photon energy is distributed between binding energy and kinetic energy; if photon energy equals binding energy, no photoelectrons are emitted.
  • An example illustrates that if photon energy is 1000 eV and binding energy is 900 eV, then kinetic energy of emitted photoelectrons would be 100 eV.

Inverse Relationship Between Binding Energy and Kinetic Energy

  • There exists an inverse relationship: as binding energy increases, kinetic energy decreases. This principle is fundamental in X-ray photoelectron spectroscopy.
  • If a core electron has a very high binding energy (e.g., thousands of eV), its kinetic energy will approach zero.

Conservation of Energy Principle

  • The attractive force between electrons and the nucleus defines their binding energies; this conservation principle explains why higher photon energies are needed to overcome higher binding energies.
  • To emit photoelectrons effectively, photon energies must exceed certain thresholds that correspond to specific electron bindings.

Practical Implications in Spectroscopy

  • The famous equation governing these relationships shows how total photon energy must equal the sum of binding and kinetic energies.
  • For lead's electronic configuration analysis via XPS spectra: only certain electron configurations appear based on their relative distances from the nucleus affecting their observable properties.

This structured overview captures essential insights into how x-ray photoelectron spectroscopy operates concerning electron behavior influenced by both binding and kinetic energies.

Understanding Binding Energy and Kinetic Energy in XPS Analysis

The Concept of Binding Energy

  • Electrons closer to the nucleus exhibit greater binding energy, while those farther away have less. This concept is crucial for understanding electron behavior in materials.
  • High binding energy electrons do not appear in lead's survey spectra because the photon energy range used is insufficient to excite these core electrons.

Kinetic Energy Graphs

  • In kinetic energy graphs, binding energy increases towards the left, indicating that 4f orbital electrons possess higher kinetic energy compared to others like 5d orbitals.
  • X-ray sources such as magnesium and aluminum are commonly used in XPS analysis, with most detectable elements having photoelectron peaks below 1200 eV.

Photoelectron Behavior

  • Photoelectrons escape from a very thin region (approximately 10 nanometers), where elastic scattering occurs. This scattering does not result in energy loss.
  • Only photoelectrons originating from this thin region contribute to XPS peaks; those from thicker regions add background noise.

Energy Conservation Principles

  • The process of photoelectron emission involves elastic scattering, ensuring total energy conservation between binding and kinetic energies.
  • Higher binding energies correlate with lower kinetic energies for emitted photoelectrons due to the distribution of photon energy between these two forms.

Practical Implications of Binding Energy Calculations

  • If an electron has high binding energy (e.g., "th"), it may yield zero photoelectron energy under certain conditions. Conversely, lower binding energies correspond to higher kinetic energies.
  • The relationship between photon energy, work function, and detected kinetic energy allows for calculating binding energies effectively using established equations.
Playlists: XPS - X-rays Photoelectron Spectroscopy
Video description

Binding Energy (BE) vs. Kinetic Energy (KE) in X-rays Photoelectron Spectroscopy (XPS) BE of electron and KE of photoelectron are inversely proposal to each other. If BE = 1000 eV, the KE must be = 0 eV, if and only if the x-rays photon Energy = 1000 eV. This supposition is based on the conservation of energy during the photoelectron generation in XPS. This is the famous photoelectric effect equation i.e., (hv = BE + KE + work function), so here the photon energy (hv) has to distribute between the BE of the electron and KE of the photoelectron. Let's prove that how energy is conserved during the photoelectron creation in XPS? Let's begin from the ejection of core electron: XPS only detects elements at a very thin region ~10 nm. In fact, x-rays (Al Kα~1486.6 eV and Mg Kα line ~1253.6 eV) can penetrate to a micro meter thick region however in this thick region, the photoelectrons lost its energy completely owing to inelastic scattering and inferior path length, thereby can not reached to the XPS detector. Just below the 10 nm region, the photoelectron may encounter a few inelastic scattering and losing some energy, however, reach to the XPS detector, but these electrons only contribute to the background of the XPS peaks. The photoelectrons escaped only from the ~10 nm will reach the XPS detector and give rise to XPS photoelectrons and Auger electron Peaks. Here, in this region the x-rays photons completely transfer its energy to an electron (elastic scattering), usually a core-level electrons are emitted. we know that in elastic scattering, no energy is lost and the total energy remains conserved in the process on photoelectron generation. (For better understanding refer to the attached image). Energy Conservation: The total energy in the photoelectron process remains constant (due to elastic scattering). The known x-rays photon energy is distributed between the BE of the electrons and KE of the photoelectrons. Inverse Relationship between KE and BE • When the BE of an electron is higher, the KE of the emitted electron is lower • When the BE of an electron is lower, the KE of the emitted electron is higher • It is TRUE because of the conservation of energy in the XPS process Using the famous equation (BE = hv- KE - ϕ), it can be shown that BE is inversely proportional to the KE of the photoelectrons. Where hv is x-rays source energy = 1486.6 eV, ϕ is work function of spectrometer = 4.6 eV, KE is the kinetic energy of the photoelectrons and measure by the XPS detector. Plugin all these value in the above equation, BE can be calculated. X-rays (hv) hv ϕ KE BE 1486.6 eV 4.6 eV 1382 eV 100 eV 1486.6 eV 4.6 eV 1282 eV 200 eV 1486.6 eV 4.6 eV 1082 eV 400 eV 1486.6 eV 4.6 eV 682 eV 800 eV 1486.6 eV 4.6 eV 482 eV 1000 eV Let's take the example of Lead (Pb) with electronic configuration: 1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p⁶4d¹⁰5s²5p⁶6s²4f¹⁴5d¹⁰6p² The electron closer to the nucleus possess higher BE as compared to far located electrons. For instance, the 4s² electrons shows the highest BE than the 4p⁶, 4d¹⁰ or 4f¹⁴ or even 5d¹⁰ (see the attached XPS survey spectrum for Pb element). Therefore, in XPS spectra, the BE increases towards the lift while the KE increases towards the right. Please subscribe to my channel and share it. Best,