2p X-ray Photoelectron Spectroscopy

• 2p XPS is a surface-sensitive technique that measures the binding energies of spin–orbit split 2p electrons to reveal elemental composition, oxidation states, and bonding characteristics.
• It employs precise energy calibration and advanced peak fitting methods, such as Voigt and Doniach–Šunjić profiles, to accurately quantify chemical shifts and resolve spin–orbit doublets.
• Applications of 2p XPS span catalysis, semiconductor analysis, and materials science, offering actionable insights into local electronic environments and surface chemistry.

2p X-ray Photoelectron Spectroscopy (XPS) is a surface-sensitive technique for interrogating the elemental composition, chemical state, and local bonding environment of atoms via analysis of the binding energies and fine structure of 2p core-level photoemission lines. This modality is distinct for its capacity to resolve spin–orbit split doublets (2p₃/₂ and 2p₁/₂), probe chemical shifts associated with oxidation or covalency, and give quantitative information about multi-element systems, including semiconductors, oxides, and transition-metal compounds (Čechal, 31 Dec 2025). The method is foundational in catalysis, materials science, and device physics, and is central to modern analyses of functional surfaces and interfaces.

1. Physical Principles and Energy Calibration

2p XPS utilizes monochromatic X-ray irradiation (typically Al Kα, Eₓ = 1486.6 eV) to eject 2p electrons from atoms within the top few nanometers of a solid. The kinetic energy of the emitted electron is given by the fundamental energy conservation equation for a metallic or grounded sample:

$$E_{\text{kin}} = E_{X} - E_{\text{CL}}^{B,F} - \phi_{\text{spectrometer}}$$

where $E_{X}$ is the X-ray energy, $E_{\text{CL}}^{B,F}$ is the core-level binding energy referenced to the common Fermi level, and $\phi_{\text{spectrometer}}$ is the work function of the analyzer (Čechal, 31 Dec 2025). The measured spectrum is commonly plotted on a binding energy (BE) scale increasing to the left, after conversion.

For insulators or semi-conducting samples subject to differential charging, an additional unknown potential must be considered; spectra are then charge-referenced either using the adventitious C 1s peak (typically set to 284.8 eV) or by deposition of metallic reference layers/low-energy electron flood guns to ensure uniform referencing (Čechal, 31 Dec 2025). The accuracy of BE calibration is paramount for quantification of chemical shifts.

2. Spin–Orbit Splitting and Doublet Structure

Atomic 2p levels exhibit spin–orbit coupling, yielding two final states: 2p₃/₂ ($j=3/2$) and 2p₁/₂ ($j=1/2$), separated by an energy $\Delta E_{so}$ whose magnitude grows rapidly with atomic number:

• Si 2p: $\Delta E_{so} \approx 0.6$ eV
• P 2p: $\Delta E_{so} \approx 0.9$ eV
• Fe 2p: $\Delta E_{so} \approx 13$ eV
• Ni 2p: $E_{X}$0 eV (Čechal, 31 Dec 2025)

According to the state multiplicity rule, the area/intensity ratio is fixed at 2:1 (2p₃/₂:2p₁/₂), an essential fitting constraint. In practice, Voigt line shapes (convolution of Gaussian broadening, instrumental and chemical, with Lorentzian natural linewidth) typically model these features. For metallic systems or when core-hole lifetimes are short, asymmetric Doniach–Šunjić shapes may improve fits (Čechal, 31 Dec 2025).

3. Chemical Shifts and Local Electronic Structure Probing

Chemical shifts (ΔE) in 2p BEs provide insights into the local valence electron density, oxidation state, and screening effects. The canonical equation is

$E_{X}$1

Positive shifts commonly accompany oxidation or decreased electronic screening, while negative shifts indicate reduction or metallization (Čechal, 31 Dec 2025). For example, the Fe²⁺ 2p₃/₂ peak in FeSe₁/₂Te₁/₂ is shifted by +1.85 eV compared with metallic Fe, reflecting increased covalency and charge localization (Awana et al., 2010).

In solid solutions such as NiWO₄–ZnWO₄, shifts in 2p BEs correlate directly with systematic changes in bond ionicity/covalency and can be disentangled from relaxation effects using the Auger parameter formalism (see section 4) (Bakradze et al., 2021). Surface- or interface-induced chemical shifts are also crucial: in GaInP/AlInP, the P 2p surface chemical shift (+0.9 eV) is diagnostic of P–P dimer formation and associated midgap states controlling band bending and Fermi-level pinning (Pour et al., 2022).

4. Quantitative Analysis and Auger Parameter

Quantification in 2p XPS relies on extraction of peak areas, cross sections, and instrument sensitivity factors. For element $E_{X}$2:

$E_{X}$3

where $E_{X}$4 is the area under the (correctly fitted) 2p peak, $E_{X}$5 is the photoionization cross section, $E_{X}$6 the IMFP, and $E_{X}$7 the overall sensitivity factor (Čechal, 31 Dec 2025).

Ambiguities from initial- and final-state effects are addressed by constructing Wagner plots and evaluating the Auger parameter:

$E_{X}$8

where $E_{X}$9 is the 2p BE and $E_{\text{CL}}^{B,F}$0 the kinetic energy of a correlated Auger electron line. Variations in $E_{\text{CL}}^{B,F}$1 directly track changes in extra-atomic relaxation (polarizability of the surrounding lattice). This methodology is essential in multicomponent or complex oxide systems, disentangling ground-state charging ($E_{\text{CL}}^{B,F}$2) from relaxation ($E_{\text{CL}}^{B,F}$3) per

$E_{\text{CL}}^{B,F}$4

and is widely applied in state-of-the-art chemical-state analysis (Bakradze et al., 2021).

5. Spectral Line Shape, Satellite Structure, and Theoretical Interpretation

Advanced line-shape analysis distinguishes main 2p peaks from charge-transfer (CT) satellites, multiplet splitting, and nonlocal screening features. In transition-metal oxides, satellites appear $E_{\text{CL}}^{B,F}$5–$E_{\text{CL}}^{B,F}$6 eV above the main line and encode the physics of local ligand screening ($E_{\text{CL}}^{B,F}$7 final states) and nonlocal screening channels via charge transfer to/off neighboring sites. Ab-initio LDA+DMFT methods, employing an Anderson-impurity Hamiltonian including full multiplet, spin–orbit, Coulomb ($E_{\text{CL}}^{B,F}$8), and hybridization physics, accurately reproduce the multi-peak envelope. The most sophisticated cluster models parameterize hybridization strengths $E_{\text{CL}}^{B,F}$9, CT energy $\phi_{\text{spectrometer}}$0, and crystal-field splitting $\phi_{\text{spectrometer}}$1 to match experiment (Ghiasi et al., 2018).

For materials with mixed valence or strong covalency, as in FeSe₁/₂Te₁/₂ or NiWO₄–ZnWO₄, deconvolution must include satellite and multiplet features; arbitrary Gaussians are insufficient, and physically motivated models, such as the Gupta–Sen or CI/LDA+DMFT, are necessary for quantitative interpretation (Bakradze et al., 2021, Awana et al., 2010, Ghiasi et al., 2018). In ultra-thin dielectrics, fitting the 2p envelope under applied voltage additionally recovers the film RC response, capacitance, and leakage via the relation $\phi_{\text{spectrometer}}$2 (Munoz et al., 2024).

6. Ultrafast and Nonlinear 2p XPS: Time and Multisite Sensitivity

Time-resolved 2p XPS (TR-XPS) extends the chemical-shift formalism to excited states, enabling ultrafast mapping of dynamic charge redistribution. In molecules, the excited-state chemical shift (ESCS) is inverted to local atomic charge via

$\phi_{\text{spectrometer}}$3

with $\phi_{\text{spectrometer}}$4 determined by ab-initio calibration; this allows direct tracking of charge dynamics at femtosecond resolution (Mayer et al., 2021). For instance, S 2p TR-XPS distinguishes $\phi_{\text{spectrometer}}$5 charge motion at S in 2-thiouracil with sub-250 fs time constants and identifies coherent electronic population oscillations.

Nonlinear, multiphoton 2p XPS at X-ray FELs produces sequential double core-hole (DCH) states—most strikingly, two-site (ts-DCH) configurations. Measurement of the tsDCH energy shift,

$\phi_{\text{spectrometer}}$6

yields a multisite-specific chemical indicator, far exceeding the sensitivity of conventional single-photon XPS: chemical shifts of $\phi_{\text{spectrometer}}$710 eV vs. $\phi_{\text{spectrometer}}$81 eV. This offers unique contrast for probing correlated valence reorganization, interatomic relaxation, and ultrafast charge transfer in complex molecules and materials (Salen et al., 2012).

7. Applications and Best Practices in 2p XPS

2p XPS is applied to:

• Surface oxidation state mapping, e.g., Fe, Ni, Cu oxidation series (Čechal, 31 Dec 2025).
• Bonding analysis in transition-metal and main-group compounds, including superconducting chalcogenides (Fe 2p in FeSe₁/₂Te₁/₂) (Awana et al., 2010), wide-gap dielectrics (Si 2p in SiO₂ thin films for capacitance/resistance extraction) (Munoz et al., 2024), and III–V semiconductor heterostructures (P 2p in GaInP/AlInP interfaces) (Pour et al., 2022).
• Delineation of local vs. nonlocal screening in correlated oxides, requiring advanced theoretical analysis for accurate peak attribution (LDA+DMFT, cluster models) (Ghiasi et al., 2018).
• Chemical-state analysis via combined photoemission–Auger parameter measurement (Wagner plots) to robustly separate initial- vs. final-state effects (Bakradze et al., 2021).

Best-practices include strict BE referencing, physical modeling of line-shape and multiplet structure, quantitative background subtraction, and cross-technique validation (e.g., complementary EXAFS, Raman, transport) (Čechal, 31 Dec 2025, Bakradze et al., 2021). Misinterpretation of BE shifts without accounting for multiplet, satellite, and charging artifacts is a major pitfall. Correct doublet separation, intensity ratio, and charge referencing are critical for the extraction of meaningful chemical/quantitative data.

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2p XPS thus constitutes a robust, chemically and electronically specific probe, continuously extended through methodological innovation (TR-XPS, nonlinear FEL-XPS, ab-initio theory) and rigorously applied analysis protocols (Čechal, 31 Dec 2025, Bakradze et al., 2021, Ghiasi et al., 2018, Pour et al., 2022, Awana et al., 2010, Munoz et al., 2024, Salen et al., 2012, Mayer et al., 2021).