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Shake - Up and Shake - off Satellite Peaks in XPS Spectra
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Shake - Up and Shake - off Satellite Peaks in XPS Spectra

What are Shakeup Satellites in XPS Spectra?

Understanding Shakeup Satellites

  • Shakeup satellites in X-ray photoelectron spectroscopy (XPS) spectra can be challenging to identify, as they often do not appear in survey spectra and require high-resolution XP spectra for detection.
  • The diagram illustrates how shakeup peaks manifest, alongside another type known as sh peaks, which may contribute to the background noise in XPS spectra.

Mechanism of Photoelectron Emission

  • When X-rays bombard a sample, core electrons are emitted, resulting in what is termed photoelectrons. This process begins with an atom that has its core electrons intact.
  • Upon ionization from X-ray bombardment, the atom loses a core electron and becomes ionized; this results in the emission of photoelectrons that generate XPS peaks.

Excitation and Energy Loss

  • As photoelectrons leave the atom, there exists a probability that they may excite valence electrons to higher energy levels during their escape.
  • The relationship between x-ray energy, kinetic energy of the photoelectron, and binding energy is crucial; when a photoelectron loses kinetic energy upon excitation, it leads to higher binding energies observed in shakeup peaks.

Ionization Phenomena

  • There’s a chance that while exiting the atom, a photoelectron can completely ionize an outermost valence electron. This phenomenon requires additional energy compared to simple excitation.
  • The decrease in kinetic energy of the outgoing photoelectron correlates with increased binding energies for shakeup peaks due to conservation of energy principles.

Characteristics of Shakeup Peaks

  • Both shakeup phenomena and satellite peaks arise from relaxation processes during XPS. The promotion of 2p electrons to 3p levels during ionization contributes significantly to these observations.
  • Shakeup satellite peaks typically appear at higher binding energies because they reflect decreased kinetic energies of emitted photoelectrons.

Conditions Favoring Visibility of Shakeup Satellites

Structural Influences on Peak Appearance

  • Aromatic structures such as benzene or polycyclic aromatic hydrocarbons exhibit more pronounced shakeup satellite peaks due to their electronic configurations.
  • Unsaturated bonds (double/triple bonds between carbon atoms), along with transition metal ions like lead or copper, also enhance visibility of these features within XPS spectra.
Playlists: XPS - X-rays Photoelectron Spectroscopy
Video description

What are Shake-up and shake-off Satellite peaks in XPS Spectra Shake- Up Satellite Peaks: In XPS, the photoionization occurs from a 1s electron and there is a possibility that a 2p electron can be promoted into a 3p level during the process of photoionization. This 2p electron is called to be "shaken-up" into the 3p. This phenomena takes a discrete energy and that energy is basically comes from the kinetic energy of the 1s photoelectron which was escaped. Resultantly, the KE of the core electron reduced by some quantized amount and the satellite peak are formed a few electron volts lower in KE (higher in BE) than the main peak. More than one shake-up satellites are also possible and the displacements and relative intensities are helpful in identifying the element’s chemical state. The band (peak) resulting from this process is referred as shake-up satellites and are always found on the high BE side of the main unrelaxed photoelectron peak. Shake-off: When the outgoing photo generated electron interacts with a valence band electrons and the valence electrons are ejected completely from the ion (to the continuum). Shake-off peaks are similar to shakeup peaks, but instead of exciting an electron to a higher state, the electron is ejected from the atom. Shake-off peaks may not appear in the spectra and sometime make the background. In this case, the KE of the photoelectron decreases more therefore, the shake-off peaks appear at higher BE. These shake-up satellite are most apparent for a system with aromatic structure (like benzene or planar structure where carbon atom form chain structure), unsaturated bond (like carbon double or triple bonds), or transition metal ions (Cu+, Cd2+, Cr3+,) Pb4+). For instance, in case of polystyrene, the π bond shake-up satellite for C 1s is about 6.7 eV higher in BE than the main photoelectron peak. The π is occupied energy level and π* is an unoccupied high level (i.e. π→π* transition). Please subscribe to my channel and shar it.