Auger Parameter Video

Video explaining the use of the Auger parameter.

Advanced Analysis of Gallium Compounds

Work by Jeremy Bourque [1] demonstrates the power of XPS to elucidate chemical trends within a series of compounds.  This work shows results from an extensive analysis of the Ga 3d5/2, Ga 2p3/2 and Ga L3M45M45 spectra of a broad series of Ga compounds.  Binding energy positions, Auger parameters and chemical state (Wagner) plots are then used to understand the trends found for the various series of related compounds and to understand the chemistry of newly synthesized complexes.  Examples of studied trends include the changes as the oxidation states goes from Ga(0) through to Ga(III), changes as the ligand is modified on Ga(I) or Ga(III) species, and trends in the various semiconducting Ga materials.  This work has been applied to more compounds in [2].


Photoelectron binding and Auger electron kinetic energies and full-width at half-maxima for high-resolution XPS spectra along with associated Auger Parameters.

  Ga 3d (left), Ga 2p3/2 (center) and Ga L3M45M45 (right) XPS spectra of Ga(m) (bottom), GaNacNacDipp (lower middle), Ga2Cl4(diox)2 (upper middle) and GaCl3 (top).



Ga 3d (left), Ga 2p3/2 (center) and Ga L3M45M45 (right) XPS spectra of GaCl3 (bottom), GaBr3 (middle) and GaI3 (top).



Wagner plot of gallium materials using Ga 3d5/2 binding energy. Symbol legend: diamond = Group 15 elements; square = Group 16 elements.

Reference:

[1] J.L. Bourque, M.C. Biesinger, K.M. Baines, Dalton Transactions, 45 (2016) 7678-7696.

[2] J.L. Bourque, R.A. Nanni, M.C. Biesinger, K.M. Baines, Inorganic Chemistry, 60 (2021) 14713-14720.

Indium 3d5/2 - M4N45N45 Auger Parameter

Indium MNN Auger spectrum.  The In M4N45N45 peak is shown with a peak fit to it. This is the more common peak to use for the calculation of the Auger parameter.

Note that the In M4N45N45 peak is the more common peak to use for the calculation of the Auger parameter.

In 3d5/2 - M4N45N45 Auger Parameter Values [1]

Reference:
[1] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R.Jr. Rumble, NIST Standard Reference Database 20, Version 3.4 (web version) (http:/srdata.nist.gov/xps/) 2003.

Auger Peaks and the Auger Parameter

Following core ionization by photoelectron emission an outer shell electron can fill the created vacancy and the energy released can result in the emission of an Auger electron. A schematic of the Auger emission process for nickel metal is presented in Figure 1. The energy of an emitted Auger electron will be equal to the emitted photoelectron binding energy (E_b(C₁)) minus the binding energy of electron that fills the vacancy in the core (E_b(C2)), minus the binding energy (in the presence of the core hole) of the level from where the Auger electron is emitted (E_b(C3)):

                 E_k ≈ E_b(C₁) – E_b(C2) – E_b(C3)               (1)

Figure 1.  Schematic of an LMM Auger electron emission from a nickel atom.

Auger spectra have unique peak shapes and positions and are useful for both elemental identification and chemical state analyses. A calculated value from both photoelectron and Auger peak positions is the Auger parameter (α). This parameter is particularly useful for chemical state analysis and can be used without interference of surface charging. Originally defined by Wagner[1,2], the Auger parameter is calculated as follows:

α = E_k(C₁C₂C₃) – E_k(C)                    (2)

where E_k(C₁C₂C₃) is the kinetic energy of the Auger transition involving electrons from C₁, C₂ and C₃ core levels and E_k(C) is the kinetic energy of the photoelectron from core level C. This form of the equation allowed for negative values of α. The Auger parameter was modified by Gaarenstroom and Winograd[3] by addition of the photon energy to α. This modified Auger parameter (α') is independent of the X-ray energy used and is calculated as follows:

α' = E_k(C₁C₂C₃) + E_b(C)                   (3)

where E_b(C) is the binding energy of the core level C. Since any surface charging shifts will be of the same magnitude, but of opposite direction in each of these two components, they will be automatically cancelled out in α'.

The graphical display (scatter plot) of the most intense photoelectron line binding energies (abscissa, oriented in the negative direction) versus the kinetic energy position of the sharpest core-core-core Auger line (ordinate) is known as a Wagner plot, chemical state plot or chemical state diagram. Positions of compounds on these plots indicate both relaxation energy and initial state effects[4,5]. Hence, the modified Auger parameter can be used in addition to the binding energy envelope to give additional insight into the shift in electronic state between transition metal compounds.

There are numerous examples of the use of Wagner plots and Auger parameter in the literature including the study of silicon/silicate materials[6,7] and TiO₂ on different supporting surfaces[8]. The NIST database[9] contains a large collection of Auger parameter values as does the Handbook of X-ray Photoelectron Spectroscopy[10].

Figure 2. Ni 2p_3/2 – Ni LMM Wagner plot for Ni metal, Ni alloys, NiO, Ni(OH)₂ and NiOOH [From 11].

In XPS spectra, measured core level binding energies, E_b, involve both the ground state and the final state relaxation energies. The response of spectator electrons to the creation of a core hole and the Auger deexcitation process causes lowering of the measured binding energy as compared to the initial state (i.e. chemical shift) binding energy and this final state relaxation energy R can vary with chemical environment. Hence, there is a need to distinguish between initial and final state contributions to the measured binding energies. It is therefore important that final state effects are correctly described if binding energy shifts are to yield useful and reliable chemical information as to the electronic structure of transition metals and their compounds. Experimentally, relaxation energy shifts are often estimated by measuring the Auger parameter shift defined by:

Δα' = ΔE_b + ΔE_k                    (4)

It is usually assumed, following the derivation by Morretti[4,5], that the relaxation energy for the doubly core-ionized state created by the Auger process, equals 2R, leading to:

Δα' ≈ 2ΔR                              (5)

In the simplest approximation used by Wagner[12] the shift in core level binding energy ΔE_b and in Auger transition kinetic energy ΔE_k are then:

ΔE_b = − Δε – ΔR                   (6)

ΔE_k = Δε + 3ΔR                     (7)

In this convention, positive values of Δε, initial state contributions, and ΔR, final state contributions, result in a shift to lower binding energy. Initial state effects, Δε, are generally understood to represent the “chemical shift” as a result of ground state electronic structure and are a function of the valence structure of the core atom, which is in turn is a function of bonding to neighboring atomic valence states. These shifts are related to the electronic states (e.g. band structures, bond directionality) and structural parameters (e.g. atomic positions, Madelung constants) of the bonded atoms.

References:
[1] C.D. Wagner, Electron Spectroscopy, in: D.A. Shirley (Ed.), Proceedings of an International Conference held at Asilomar, Pacific Grove, California, USA, 7-10 September, 1971, North-Holland, Amsterdam, 1972, p. 861
[2] C.D. Wagner, Anal. Chem. 44 (1972) 967.
[3] S.W. Gaarenstroom, N. Winograd, J. Chem. Phys. 67 (1977) 3500.
[4] G. Moretti, The Auger Parameter, in: D. Briggs, J.T. Grant (Eds.), Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, IM Publications, Chichester, UK, 2003, pp. 501-530.
[5] G. Moretti, J. Electron Spectrosc. Relat. Phenom. 95 (1998) 95.
[6] P.S. Arora, R.St.C. Smart, Surf. Interface Anal. 24 (1996) 539.
[7] M. Stevenson, P.S. Arora, R.St.C. Smart, Surf. Interface Anal. 26 (1998) 1027.
[8] J.A. Mejías, V.M. Jiménez, G. Lassaletta, A. Fernández, J.P. Espinós, A.R. Gonzálex-Elipe, J. Phys. Chem. 100 (1996) 16255.
[9] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R. Jr. Rumble, NIST Standard Reference Database 20, Version 3.4 (Web Version) (http:/srdata.nist.gov/xps/) 2003.
[10] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp, Eden Prairie, MN, 1992.
[11] M.C. Biesinger, L.W.M. Lau, A.R. Gerson, R.St.C. Smart, Physical Chemistry Chemical Physics, 14 (2012) 2434.
[12] C.D. Wagner, J.A. Taylor, J. Electron Spectrosc. Relat. Phenom. 28 (1982) 211.

Auger Process and Notation

The Auger process involves three steps:
1) A core electron is removed by the photoelectric process
2) An electron from a higher orbital falls down almost immediately to fill this hole (a radiationless transition)
3) Excess energy of the exited state ion is removed by the ejection of an Auger electron.

A schematic of this process for a KL2L3 transition is shown in Figure 1. Nomenclature used for the description of Auger transitions are presented in Table 1. Refer to the reference provided for a more in-depth description. Figure 1. Schematic showing the three steps involved in the Auger process. The KL2L3 Auger transition is illustrated. The open circles symbolize holes (absence of electrons) [1].

Table 1. X-ray notation of electron energy states [1].

The higher transitions are O1 = 5s2 etc., P1 = 6s2 etc. Q1 = 7s2 etc.

Reference:

[1] Richard P. Gunawardane and Christopher R. Arumainayagam in "Handbook of Applied Solid State Spectroscopy" Chapter 10, Auger Electron Spectroscopy, Springer, pp 451 - 483 (2006).

Cadmium Auger Parameters

A table of values for the Cd 3d5/2 - Cd M4N45N45 modified Auger parameter is given below. See the cadmium metal file and the figure below for position of the Cd M4N45N45 Auger peak.

Cd 3d5/2 - Cd M4N45N45 Auger parameter values [1].

Cd MNN spectrum (Al X-ray source) of cadmium metal showing the position of the Cd M4N45N45 peak.

Reference:
[1] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R.Jr. Rumble, NIST Standard Reference Database 20, Version 3.4 (web version) (http:/srdata.nist.gov/xps/) 2003.

Modified Auger Parameter Defined

The modified Auger parameter can be used to follow systematic changes in the final state effects without interference of surface charging. Originally defined by Wagner [1,2], the Auger parameter is calculated as follows:

α = Ek(C1C2C3) – Ek(C)

where Ek(C1C2C3) is the kinetic energy of the Auger transition involving electrons from C1, C2 and C3 core levels and Ek(C) is the kinetic energy of the photoelectron from core level C. This form of the equation allowed for negative values of α. The Auger parameter was modified by Gaarenstroom and Winograd [3] by adding the photon energy to α. This modified Auger parameter (α΄) is independent of the X-ray energy used and is calculated as follows:

α΄= Ek(C1C2C3) + Eb(C)

where Eb is the binding energy of the core level C. Since any surface charging shifts will be the same magnitude, but of opposite direction in each of these two components, it is automatically cancelled in α΄.

The graphical display (scatter plot) of the most intense photoelectron line binding energies (abscissa, oriented in the negative direction) versus the kinetic energy position of the sharpest core-core-core Auger line (ordinate) is known as a Wagner plot, chemical state plot or chemical state diagram. Positions of compounds on these plots indicate both relaxation energy and initial state effects [4]. Hence, the modified Auger parameter can be used with the BE envelope to give additional insight to the shift in oxidation state between TM compounds.

There are numerous examples of the use of Wagner plots and the Auger parameter in the literature including the study of silicon/silicate [5,6] and TiO2 on different supporting surfaces [7]. The NIST database [8] contains a large collection of Auger parameter values.

References:
[1] C.D. Wagner, Electron Spectroscopy, in: D.A. Shirley (Ed.), Proceedings of an International Conference held at Asilomar, Pacific Grove, California, USA, 7-10 September, 1971, North-Holland, Amsterdam, 1972, p. 861
[2] C.D. Wagner, Anal. Chem. 44(6) (1972) 967-973.
[3] S.W. Gaarenstroom, N. Winograd, J. Chem. Phys. 67(9) (1977) 3500-3506.
[4] G. Moretti, The Auger Parameter, in: D. Briggs, J.T. Grant (Eds.), Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, IM Publications, Chichester, UK, 2003, pp. 501-530.
[5] P.S. Arora, R.StC. Smart, Surf. Interface Anal., 24 (1996) 539-548.
[6] M. Stevenson, P.S. Arora, R.StC. Smart, Surf. Interface Anal. 26 (1998) 1027-1034.
[7] J.A. Mejías, V.M. Jiménez, G. Lassaletta, A. Fernández, J.P. Espinós, A.R Gonzálex-Elipe, J. Phys. Chem. 100 (1996) 16255-16262.
[8] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R. Jr. Rumble, NIST Standard Reference Database 20, Version 3.4 (Web Version) (http:/srdata.nist.gov/xps/) 2003.