(http://www.chem.qmw.ac.uk/surfaces/scc/)
As it can be seen, titanium exhibits very large chemical shifts between different oxidation states of the metal.
Quantification
The spectrum obtained is a plot of the number of detected electrons per energy interval versus their kinetic energy. Each element has a unique spectrum. If there is a mixture of elements being analysed, the mixture of elements is approximately the sum of the peaks of the individual constituents (http://www.sogang.ac.kr/~ nlopt/seminar/ XPS-YHS.ppt, 5/10/03). Quantitative data can therefore be obtained from peak height or peak areas. Identification of the chemical states can also be achieved by making exact measurements of peak positions and separations.
Initial State effects
The initial state is the ground state of an atom prior to the photoemission process. If the atom’s initial energy state is changed (by making a chemical bond etc) then the binding energy of electrons will change. All core levels binding energies for an element will undergo the same chemical shift. The initial state effects are responsible for observed chemical shifts.
The initial state effects are those factors that influence the charge state of an atom before the photon strikes it. The causes of the initial state effects involve any change in bonding of an atom that changes the binding energy of the electron. This will cause a shift in the peak position. Factors include changes in –
- Hybridisation.
- Oxidation state (Binding energy increases as oxidation state increases).
- Degree of polar covalent or ionic bonding (as atoms lose valence electrons, the binding energy of all remaining electrons increases).
(http:www.nottingham.ac.uk/~ppzpjm/sect6_1.htm, 6/10/03).
Koopmans’ Theorem
Koopmans’ theorem is an approximation used to interpret photoelectron spectra. According to Koopmans’ theorem “the binding energy measured by XPS should be equal to the calculated orbital energy”, (Bundle, C. R. & Baker, A. D., 1981, p4).
There are numerous assumptions made in order for the approximation to be used to interpret photoelectron spectra -
-
The frozen orbital approximation. This states that electron distribution is the same in M+ as in M. “Molecular orbitals appropriate for the parent molecule will be the same as those for the ionized molecule”, (Drago, R. S., 1977, p571). The loss of the electron causes no disturbance in the other orbitals.
-
Relativistic effects are the same in both M+ and M.
(Coughlin, M., 2002, p23).
Koopmans’ theorem is therefore used to explain that the peaks in a photoelectron spectrum can be associated with different molecular orbitals in the parent orbital.
The Koopmans’ theorem can be seen as a bad approximation since it neglects the relaxation effects in the photo ion and the difference in the correlation before and after photoionisation. If there is any electronic relaxation (a change in the molecular orbitals of an ionized molecule because of a change in electronic repulsions), or if there is a change in correlations, then the Koopmans’ theorem breaks down. Electrons in the vicinity of the positive charge will rearrange to screen it i.e. reduce its energy. The energy reduction is called the relaxation energy and can originate both from the electrons on the atom containing the core-hole (intra-atomic screening) and from those on surrounding atoms (interatomic screening). Relaxation/screening is thus a final state effect.
Final State Effects
Final state effects are those factors that influence the charge state of an atom after the photon has it, or has affected the photoelectron while it is leaving. There are a number of different final state effects and these are discussed below.
Relaxation
The photoemission event leaves a hole in the core level. The localized hole can dissipate and become delocalized due to inflow (diffusion) of charge. The process of charge diffusion is called relaxation. There are two different types of relaxation. During a process known as intra-atomic the core hole is delocalized due to rearrangement of electrons in the orbitals of the excited atom. Inter-atomic relaxation involves the core hole being delocalized due to movement of electrons from the surrounding atoms in the material.
As relaxation causes the localized core hole to become more diffuse, the leaving electron can escape at a higher kinetic energy. This therefore increases the extent of relaxation and causes the binding energy of the electron to decrease. The effect of increasing relaxation causes a shift in peak position to a lower binding energy.
Relaxation will not cause an extra peak to appear in the spectra. It can only be observed by comparing cases with and without relaxation.
Satellites
The photoemission event causes the neutral atom to become excited (charged). This excited state can return to the ground state through rearrangement of its orbitals to an intermediate state. The photoelectron will appear as though it has left from an intermediate (satellite) state rather than from the excited (or relaxed) state.
(, 6/10/03)
The Shake up process
In the simple case of photoejection, a core electron leaves the atom, having had the entire energy of the photon. It is focused by the spectrometer and appears as a single spectra line at kinetic energy KE. The loss of a core electron by photoemission increases the nuclear charge. This major perturbation gives rise to substantial reorganization of the valence electrons. It can involve excitation of one of the electrons to a higher unfilled level, and lose an amount of kinetic energy equal to the excitation energy. This is referred to the shake up process.
The Shake off process
In a process similar to shake up, valence electrons can be completely ionized. The departing photoelectron has transferred sufficient energy to the valence electron to move it entirely from the atom. This process, referred to as shake off, leaves an ion with vacancies in both the core level and the valence level.
Effects of the Shake up process and the Shake off process
The energy used for the shake up process is drawn from the kinetic energy of the photoelectron. Therefore an electron that leaves form an orbital that undergoes shake up and shake off will appear to be from the rehybridized orbital, (/~ppzpjm/sect6_1.htm, 10/10/03). The shake up process always leads to higher binding energy configerations. The effect can cause a new peak (sattelite) to appear.
Plasmon losses
As the electron leaves, it may lose energy to the surrounding free electrons in the material through collective electron-electron interactions. These collective losses are called plasmon interactions (a plasma is a phase of charged particles). The loss interactions can occur with bulk states or surface states of the free electron sea. ‘Both surface and bulk plasmons contain an intrinsic and extrinsic component. Intrinsic losses occur simultaneously with photoionisation due to the coupling between the core hole and the plasmon field. Extrinsic losses occur due to secondary interactions with the surface after emission from the atom’, (O’Shea, J. N., p19, 1998).
A plasmon loss causes the electron to lose its kinetic energy. It therefore appears to gain binding energy. This leads to additional peaks at higher binding energies. The plasmon loss electrons must still be counted as part of the original photoemission event. An electron that is scattered inelastically becomes part of the background signal, (www.augustus.scs.uiuc.edu/nuzzogroup/ PPT/XPS% 20Class%2099 .PPT, 10/10/03).
The spectra of free electron metals will contain pronounced electron energy-loss peaks due to plasmon excitations, (Kurth, M. and Graat, P. C. J., 2002). The relative position of a plasmon loss peak in an XPS spectrum can be determined by looking at electron scattering spectra from the material. The XPS core levels should have a plasmon loss peak at a higher binding energy.
Spin orbit coupling
This factor can appear often in an XPS spectra. It occurs when an electron has a spin s of ½. Each orbital has an angular momentum of 1. The vector sum will then be 1+/- s. This leads to splitting in two distinct energy levels for all except the s orbitals. Two components will be observed in an XP spectrum, separated in a binding energy by an amount referred to as the spin orbit splitting, (Carley, A. F., 1980, p9).
Multiplet splitting
This process occurs when an unpaired valence electron interacts with the unpaired core hole electron which has been left after photoemission. This results in two final states which give two peaks in the XP spectrum. “Multiplet splitting is most pronounced for 3s orbitals when unpaired electrons are present in the 3d orbitals”, (O’Shea, J. N., 1998, p20).
The core hole
After a core electron is ejected, the ionized atom is in a high excited state. The core hole will then decay via the two following processes –
- Auger transition
- X-ray fluorescence
Auger transition
In the Auger process, one electron falls from a higher level to fill an initial core hole in the K-shell and the energy liberated in this process is simultaneously transferred to a second electron. A fraction of this energy is required to overcome the binding energy of this second electron and the remainder is retained by this emitted Auger electron as kinetic energy, (http://www-ssrl.slac.stanford.edu/nilsson group/ pages / core _spec_ xps.html#XES, 7/10/03).
In general, since the initial ionization is non-selective and the initial hole may therefore be in various shells, there will be many possible Auger transitions for a given element - some weak and some strong in intensity. Auger Spectroscopy is based upon the measurement of the kinetic energies of the emitted electrons which are independent of the mechanism of initial core hole formation. Each element in a sample will give rise to a characteristic spectrum of peaks at various kinetic energies.
X-ray fluorescence
Transition of the outer electron to the core hole releases energy in the form of an x-ray photon. The released photon is equal in energy to the differences between energy levels A and B.
(Coughlin, M., 2002, p17).
Such core hole decay processes are very rapid, and a typical lifetime of core holes is in the order of a few femtoseconds (10-15 s), (http://fysik5.fysik.uu.se/research/molecules/molecules.html, 7/10/03).
References
Bundle, C. R. & Baker, A. D., (1981) Electronspectroscopy, Theory, Techniques and
Applications. Academic Press: London, New-York, Toronto, San Francisco and
Sydney.
Carley A. F. (1980) Electron Spectroscopic studies of solid surfaces, PhD Thesis.
Carlson, T. A., (1975) Photoelectron and Auger Spectroscopy. Plenum Press: New-
York and London.
Coughlin, M., (2002) An XPS/STM study of the Chemistry of small molecules on
bulk and Nano-Particulate Copper. Unpublished PhD Thesis, University of Cardiff.
Drago, R. S. & Saunders, W. B., (1977) Physical Methods in Chemistry. Company:
Philadelphia, London and Toronto.
Hollas, J. M., (1998) High resolution spectroscopy. 2nd edition. John Wiley and Sons:
Chichester, New-York, Weinheim, Brisbane, Singapore and Toronto.
Ibach, H. & Roy D., (1977) Topics in Current Physics. Springer-Varleg: Berlin,
Heidelberg and New-York.
Kurth, M. Graat, P. C. J., (2002) Surface and Interface Analysis. Max Planck Institute
for Metal research, Stuttgart, Germany.
O’Shea, J. N., (1998) X-Ray Photoelectron Spectroscopy and Reactivity of Alkali-
Doped Ni (110)-0 surfaces, Unpublished PhD Thesis, University of Cardiff.
Websites used :
www.geocities.com/capecanaveral/6367/esca1.htm, [Accessed 3/10/03].
http://www.sogang.ac.kr/~ nlopt/seminar/ XPS-YHS.ppt, [Accessed 5/10/03].
http:www.nottingham.ac.uk/~ppzpjm/sect6_1.htm, [Accessed 6/10/03].
, [Accessed 6/10/03].
www.augustus.scs.uiuc.edu/nuzzogroup/ PPT/XPS% 20Class%2099 .PPT, [Accessed 10/10/03].
http://www-ssrl.slac.stanford.edu/nilsson group/ pages / core _spec_ xps.html#XES, [Accessed 7/10/03].
http://fysik5.fysik.uu.se/research/molecules/molecules.html, [Accessed 7/10/03].
