XPS & ESCA Surface Analysis Lab
Model X-ray Photoelectron Spectroscopy (XPS / ESCA), core-level photoemission ($E_K = h\nu - E_B - \Phi_{\text{spec}}$), chemical oxidation state shifts, spin-orbit multiplet splitting, Shirley inelastic background subtraction, and contract testing facility DCF.
Spectrometer Parameters
Surface Lab Commercial DCF
Wide Survey Spectrum (0 – 1200 eV Binding Energy)
Elemental IdentificationHigh-Resolution Core-Level Voigt Deconvolution
Oxidation State Analysis10-Year Surface Characterization Laboratory DCF Schedule
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Engineering & Surface Chemistry Guide
1. The Photoelectric Law & Binding Energy Calibration
XPS relies on the Einstein photoelectric effect under ultra-high vacuum ($<10^{-9}\ \text{mbar}$). Monochromatic $\text{Al } K\alpha$ soft X-rays ($h\nu = 1486.67\ \text{eV}$) penetrate the sample, ionizing core electrons with kinetic energy: $$E_K = h\nu - E_B - \Phi_{\text{spec}}$$ where $\Phi_{\text{spec}}$ is the spectrometer work function. Spectra are calibrated against adventitious carbon ($\text{C } 1s = 284.8\ \text{eV}$) or the Fermi edge of sputter-cleaned noble metals.
2. Chemical Shifts & Formal Oxidation States
The binding energy of a core electron reflects the electrostatic potential of its nuclear charge shielded by surrounding valence electrons. Oxidation extracts valence charge, reducing screening and shifting core levels toward higher binding energy: $$\Delta E_B = k \cdot q_i + \sum_{j \ne i} \frac{q_j}{R_{ij}}$$ In silicon microelectronics, $\text{Si}^0$ metal ($99.3\ \text{eV}$) shifts upward by $+3.8\text{–}4.0\ \text{eV}$ in stoichiometric $\text{SiO}_2$ ($\text{Si}^{4+} = 103.3\ \text{eV}$), with intermediate suboxides ($\text{Si}^+$, $\text{Si}^{2+}$, $\text{Si}^{3+}$) indicating atomic interfacial roughness.
3. Spin-Orbit Coupling & Multiplet Splitting
Non-zero orbital angular momentum states ($l > 0$) couple with electron spin ($s = \pm 1/2$) to split core peaks into doublets with quantum total angular momentum $j = l \pm 1/2$. The peak area ratio is strictly determined by state degeneracies $(2j + 1)$: $$p_{1/2} : p_{3/2} = 1:2 \quad (l=1), \quad d_{3/2} : d_{5/2} = 2:3 \quad (l=2), \quad f_{5/2} : f_{7/2} = 3:4 \quad (l=3)$$ For titanium $\text{Ti } 2p$, spin-orbit splitting creates a $5.7\ \text{eV}$ separation between $\text{Ti } 2p_{3/2}$ ($458.8\ \text{eV}$) and $\text{Ti } 2p_{1/2}$ ($464.5\ \text{eV}$) in passivating $\text{TiO}_2$.
4. Inelastic Mean Free Path & Angle-Resolved Profiling (ARXPS)
Although soft X-rays penetrate micrometers into solids, photoelectrons have an Inelastic Mean Free Path (IMFP, $\lambda$) of only $1\text{–}3.5\ \text{nm}$. Over $95\%$ of all detected electrons emerge from an escape depth $d \approx 3\lambda \cos\theta$. By tilting the sample from surface normal ($\theta = 90^\circ$) to grazing emission ($\theta = 15^\circ$), vertical sampling depth drops from $8\ \text{nm}$ to under $2\ \text{nm}$, isolating the outermost ultrathin native oxides and surface contamination layers without destructive ion beam sputtering.
Frequently Asked Questions
XPS & ESCA Self-Grading Quiz
1. What happens to the measured core-electron binding energy $E_B$ when an atom transitions to a higher positive oxidation state?
2. For a $2p$ core level split into $2p_{1/2}$ and $2p_{3/2}$ doublets by spin-orbit coupling, what is the theoretical peak area ratio?
3. In Angle-Resolved XPS (ARXPS), why does tilting the specimen to a grazing take-off angle dramatically enhance surface sensitivity?