Initial vs Final State Effects #
In X-ray Photoelectron Spectroscopy (XPS), core-level binding energy shifts are influenced by both initial and final state effects.
Initial state effects are related to the ground state properties of the atom before core ionization. They reflect the static electronic environment and the potential experienced by core electrons in their initial state.(1)
Final state effects are related to the response of the system to the creation of a core hole. They are associated with the polarization of the electronic cloud of the atom after it has been ionized by X-ray irradiation. Final state effects involve the rearrangement or relaxation of the electronic structure in response to the creation of the core hole. These are dynamic effects that occur after photoemission.
Timing: Initial state effects relate to the electronic structure before the photoemission event, while final state effects occur after the core hole has been created.
Nature: Initial state effects are static, representing the ground state, while final state effects are dynamic, reflecting the system’s response to the core hole.
Charge Density: Initial state effects are based on the initial state charge density, while final state effects are due to the difference in final state charge density that can’t be accounted for by the initial state difference.
Observable: Photoelectron spectroscopy measures the final state of the system. Therefore, both initial and final state effects are ultimately manifested in the final state. However, initial state effects can be thought of as a part of the screening that was already present in the ground state.(2)
Changes in Local Valence Charge:
The local valence charge of an atom in a molecule or solid directly affects the potential experienced by its core electrons. For example, a change in the charge distribution of an atom in molecules or solids will change the potentials of the core electrons. Differences in the valence charge due to chemical bonding can cause a shift in core-level binding energies. The direction of the charge flow also impacts the potential and core-level binding energy.(3)
Madelung Potential:
In ionic solids, the Madelung potential, which is the electrostatic potential due to the surrounding ions, contributes to initial state effects. The Madelung potential at the core-ionized ion can be positive or negative, depending on whether the ion is negative or positive, respectively. The Madelung potential will reduce the magnitude of the core-level shift predicted by only considering the local charge. The Madelung potential depends on the effective charge of an ion.(3)
Chemical Bonding and Structure:
The nature of the chemical bonds in which an atom is involved influences the potentials of core electrons. This is because the charge distribution of the atom is changed through bonding. The local structure of the material, including the atomic positions of neighbouring atoms, affects the core-level binding energies. For example, in nickel compounds, initial state effects are related to the electronic states (band structures, bond directionality) and structural parameters (atomic positions, Madelung constants) of the bonded atoms.(4)
Surface Effects:
The presence of an atom at a surface can cause a change in the core-level binding energy compared to the bulk due to a difference in the initial state. The narrowing of the valence band at the surface of a metal is an initial-state effect that leads to a shift in core-level binding energy. This occurs because the narrowing of the band leads to a net negative charge on the surface atoms. Adsorption of an atom in a surface electrostatic dipole layer, and surface charging of an insulating sample are also examples of this.
Charge Transfer
Charge transfer between a supported nanoparticle and a support is an initial state effect that can influence core-level binding energies. For example, charge transfer from an alkali layer to a transition metal surface would shift the core levels of the top layer of transition metal atoms to smaller binding energy.
When a metal is deposited on an oxide surface, the core level binding energy of the metal will be shifted due to charge transfer with the oxide.(5)
Band Bending:
Band bending at semiconductor surfaces and interfaces is an initial state effect that changes the electrostatic potential at the atom and shifts core-level binding energies.(2)
Substituent Effects:
The ability of a substituent to create a positive potential at a nearby site influences the initial-state effects.(6)
Hybridization:
Changes in the hybridization of orbitals can affect the core level binding energies through initial state effects. For example, the amount of 2s to 2p hybridization in nitrogen influences the N 1s binding energy in molecules.(3)
Work function changes:
Work function changes can also influence the initial state effects through their impact on the Fermi level. However, it is important to note that the work function is an extrinsic property and is not considered a ground-state effect.(7)
Electrostatic potential changes:
Anything that changes the electrostatic potential at the atom under study takes the form of an initial-state contribution.(2)
Multiplet splitting:
Multiplet splitting. In open-shell systems, the coupling of the core hole with the unpaired valence electrons can lead to multiple final states with slightly different energies. This results in a splitting of the core-level peak into several components. The relative intensities of these multiplets are determined by angular momentum coupling rules and can be modified by ligand field effects and covalency. For example, the 2p XPS of Fe2O3 shows a complex structure due to multiplet splitting and covalent mixing. In the case of the Mn 2p XPS, the 2p3/2 peak shows multiplet splitting while the 2p1/2 peak is broadened due to many-body effects.
Plasmon loss:
Plasmon losses are a final state effect in which the outgoing photoelectron loses energy by exciting collective oscillations of the valence electrons (plasmons). These appear as characteristic energy loss features.
Shake-up and Shake-off:
Shake-up and shake-off effects occur when the outgoing photoelectron excites a valence electron to a higher energy level. This process results in a loss of kinetic energy by the photoelectron and the appearance of satellite peaks at higher binding energies. Shake-up involves excitation to a bound state, while shake-off involves excitation to an unbound state.(8)
Differential broadening:
Coster-Kronig transitions can affect different core levels differently. For example, the Ni 2p1/2 region is observed to be broader than the Ni 2p3/2 region because the 2p1/2 core hole can decay through Coster-Kronig transitions, while the 2p3/2 cannot.(9) Similarly, the Ti(2p) XPS of Ti(IV) shows that the Ti(2p1/2) peak is broader than the Ti(2p3/2) peak, which is attributed to a larger number of decay channels for the 2p1/2. It has also been suggested that this difference in broadening is not due to a shorter lifetime of the 2p1/2 core hole but to the superposition of intensities of several unresolved final states. The 2p1/2 core-hole has a shorter lifetime because there are more decay channels available to it.(10)
Vibrational broadening:
Vibrational excitations can occur in the XPS excitation to ionic states, and these excitations lead to a broadening of the observed XPS peaks. This broadening is often described as Franck-Condon (FC) broadening. Because these excitations happen as the system transitions to its final state, this is also a final state effect.
Core-hole screening:
When a core electron is ejected in XPS, a core hole is created. This core hole is a highly positive charge, and the system responds to screen this charge. Charge transfer in the final state is the movement of electrons towards the core-ionized atom to screen this positive charge. This screening process is a final state effect that reduces the energy required to create a core hole. The amount of charge transfer in the final state can vary depending on the chemical environment of the atom.
Ligand to Metal Charge Transfer:
In some cases, core-level shifts can be attributed to ligand-to-metal charge transfer in the final state. This type of charge transfer occurs when an electron moves from an orbital that is primarily on a ligand to one that is primarily on a metal or cation.
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- Egelhoff Jr, W. F. “Core-level binding-energy shifts at surfaces and in solids.” Surface Science Reports 6.6-8 (1987): 253-415. Read it online here.
- Bagus, Paul S., et al. “Mechanisms responsible for chemical shifts of core-level binding energies and their relationship to chemical bonding.” Journal of electron spectroscopy and related phenomena 100.1-3 (1999): 215-236. Read it online here.
- Biesinger, Mark C., et al. “The role of the Auger parameter in XPS studies of nickel metal, halides and oxides.” Physical Chemistry Chemical Physics 14.7 (2012): 2434-2442. Read it online here.
- Pacchioni, Gianfranco, and Hans-Joachim Freund. “Controlling the charge state of supported nanoparticles in catalysis: lessons from model systems.” Chemical Society Reviews 47.22 (2018): 8474-8502. Read it online here.
- Hohlneicher, George, Harald Pulm, and Hans-Joachim Freund. “On the separation of initial and final state effects in photoelectron spectroscopy using an extension of the auger-parameter concept.” Journal of electron spectroscopy and related phenomena 37.2 (1985): 209-224. Read it online here.
- Thøgersen, Annett, et al. “An experimental study of charge distribution in crystalline and amorphous Si nanoclusters in thin silica films.” Journal of Applied Physics 103.2 (2008). Read it online here.
- Bagus, Paul S., Eugene S. Ilton, and Connie J. Nelin. “The interpretation of XPS spectra: Insights into materials properties.” Surface Science Reports 68.2 (2013): 273-304. Read it online here.
- Baer, Donald R., et al. “Practical guides for x-ray photoelectron spectroscopy: First steps in planning, conducting, and reporting XPS measurements.” Journal of Vacuum Science & Technology A 37.3 (2019). Read it online here.
- Bagus, Paul S., et al. “A new mechanism for XPS line broadening: the 2p-XPS of Ti (IV).” The Journal of Physical Chemistry C 123.13 (2018): 7705-7716. Read it online here.
