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Why d - orbital has a doublet in XPS Spectra - X-rays Photoelectrons Spectroscopy
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Why d - orbital has a doublet in XPS Spectra - X-rays Photoelectrons Spectroscopy

XPS Analysis of Orbital Peaks

Understanding XPS Peaks for Orbitals

  • In X-ray Photoelectron Spectroscopy (XPS) analysis, there is a distinction between peaks for different orbitals. A single peak represents 's' orbitals, while two peaks correspond to 'd' orbitals, specifically labeled as 3/2 and 5/2.
  • The nomenclature used in XPS refers to the principal quantum number; here it is three. This indicates the energy level of the electrons being analyzed.
  • The term "angular momentum" relates to the orbital angular momentum associated with these electrons. For 'd' orbitals, this value is set at two.

Spin Angular Momentum Contribution

  • The discussion includes spin angular momentum, which can take values of +1/2 or -1/2. This aspect is crucial in determining the total angular momentum observed in the peaks.
  • When combining orbital angular momentum with spin, if both are aligned (spin up), it results in a total angular momentum of 5/2, leading to one peak. Conversely, when spin is down (subtracting from orbital), it results in a total of 3/2 for another peak.
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Why p, d, and f orbitals have double peaks in XPS Spectra? These double peaks are called Multiplet splitting or Doublet or spin-orbit splitting One line answer is when there is *unfilled shells* containing *unpaired electrons*. For example, Mn²⁺= 1s²2s²2p⁶3s²3p⁶3d⁵4s² (where in 3d⁵, all five electrons are unpaired and with parallel spins, here we get doublet for 5d orbital like Mn 3d3/2 & Mn 3d5/2) Let's explain it in detail If the orbital angular momentum (𝑙) = 0, we get single XPS peaks like for s- orbitals such as 1S, 2S, 3S, 4S..... If 𝑙 greater than 0, a doublet XPS peak, which means 𝑙 =1, p-orbitals, 𝑙 =2, d-orbitals, 𝑙 =3, f-orbitals 𝑥n𝑙j nomenclature for XPS doublet peaks like (Zn 2p1/2 & Zn 2p3/2), (Ag 3d3/2 & Ag3d5/2), (Pb 4f5/2 & Pb 4f7/2) 𝑥 – represents elements such as Co, Fe, Ti, Zn, Cu, Y, Mn,..... n : principle quantum number, 1,2,3,4.... 𝑙 : orbit angular momentum quantum number j : total angular momentum quantum number; j = 𝑙 ± s (where s =±1/2 is spin angular momentum) _______________________________ For p-orbital: For p-orbital, 𝑙 =1, n = 2, then j = 𝑙 + s = 1+1/2 = 3/2 ( 2p3/2) j = 𝑙 -s = 1-1/2 = 1/2 (2p1/2) ______________________________ For d-orbital: For d-orbital, 𝑙 =2, n = 3, then j = 𝑙 + s = 2+1/2 = 5/2 ( 3d5/2) j = 𝑙 -s = 2-1/2 = 3/2 (3d3/2) ________________________________ For f-orbital: For f-orbital, 𝑙 =3, n = 4, then j = 𝑙 + s = 3+1/2 = 7/2 ( 4f7/2) j = 𝑙 -s = 2-1/2 = 3/2 (4f5/2) Source: Handbook of X-ray Photoelectron Spectroscopy by John F. Moulder