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] Wagner, CD 1972, Electron Spectroscopy, Proceedings of an International Conference held at Asilomar, Pacific Grove, California, USA, 7-10 September, 1971. ed. DA Shirley, North-Holland, Amsterdam, The Netherlands, pp. 861.
[2] Wagner, CD 1972, ‘Auger lines in X-ray photoelectron spectrometry’, Analytical Chemistry, vol. 44, no. 6, pp. 967-973.
[3] Gaarenstroom, SW & Winograd, N 1977, ‘Initial and final state effects in the ESCA spectra of cadmium and silver oxides’, Journal of Chemical Physics, vol. 67, no. 9, pp. 3500-3506.
[4] Moretti, G 2003, Surface Analysis by Auger and X-ray Photoelectron Spectroscopy ed. D Briggs & JT Grant, IM Publications, Chichester UK, pp. 501-530.
[5] Arora, PS & Smart, RStC 1996, ‘Formation of silicate structures in oxidized nickel surfaces using low-temperature plasma reaction’, Surface and Interface Analysis, vol. 24, pp. 539-548.
[6] Stevenson, M, Arora, PS & Smart, RStC 1998, ‘XPS studies of low temperature plasma-produced graded oxide-silicate-silica layers on titanium’, Surface and Interface Analysis, vol. 26, pp. 1027-1034.
[7] Mejías, JA, Jiménez, VM, Lassaletta, G, Fernández, A, Espinós, JP & Gonzálex-Elipe, AR 1996, ‘Interpretation of the binding energy and Auger parameter shifts found by XPS for TiO2 supported on different surfaces’, Journal of Physical Chemistry, vol. 100, pp. 16255-16262.
[8] Wagner, CD, Naumkin, AV, Kraut-Vass, A, Allison, JW, Powell, CJ & Rumble, JR Jr. 2003, NIST Standard Reference Database 20, Version 3.4 (Web Version) .