Principles of photoelectron spectroscopy

The purpose of this chapter Is to discuss the principles of photoelectron spectroscopy and Its applications In the semiconductor and microelectronics Industries. Other recent reviews (5-10) have dealt with the use of XPS, AES, SIMS and ISS for failure analysis and materials characterization for these and related Industries. 

The principles of photoelectron spectroscopy have been discussed extensively in recent publications. Consequently, we give only a very brief introduction to the subject, with emphasis on aspects we consider particularly relevant in relation to organic sulfur compounds. For detailed treatments we refer to the classical book by Turner and his co-workers and to the recent volume edited by Brundle and Baker. The latter contains numerous references to relevant books, reports and review articles. 

The theoretical and experimental principles of PE spectroscopy have been reviewed extensively7-10. In particular, the reader is referred to the chapter The Photoelectron Spectra of Saturated Hydrocarbons in the volume The Chemistry of Alkanes and Cycloalkanes of the present series11. Consequently we shall limit ourselves to the essentials needed for following the arguments presented in this chapter. 

Figure 9. Principle of NeNePo spectroscopy. The probe pulse detaches the photoelectron from the negative ion, introducing a vibrational wavepacket into the ground state of the neutral particle. Its propagation is interrogated by the probe pulse, which ionizes it to a positive ion.

Here we survey a series of possible surface-sensitive measurements which in principle can be used to study the surfaces of conjugated polymers and the early stages of metal interface formation. We then motivate the use of photoelectron spectroscopy. 

The reaction AB —) I hv AB e is the basis of photoelectron spectroscopy and photodetachment methods. Many precise and accurate ionization potentials of molecules have been obtained by studying the photoionization of neutral molecules. The same principles apply to the photon methods for determining electron affinities, except that negative ions are studied. The electron affinities of over 1,000 atoms, radicals, clusters, and small molecules have been determined using... 

The meaning of the quantities evaluated in this way is quite clear. With each occupied one-electron wave-function is associated an energy s which for closed shell systems represents the ionisation potential from that level s) and this ionisation potential is, using the technique of photoelectron spectroscopy (93) measurable in principle and often, already in practice. The total energy can also be calculated, and represents the energy of formation of the system from infinitely separated nuclei (or nuclei with cores) and electrons. Net orbital populations, bond populations and gross populations are readily defined. 

This chapter is directed toward both new students and established researchers in organometallic chemistry whose expertise is not in the area of photoelectron spectroscopy. It is recognized that relatively few laboratories have direct access to photoelectron instrumentation, but that many are interested in the information that can be obtained. This chapter will briefly describe the photoelectron experiment and general sample requirements, and the principles for understanding the information contained in the data. The presentation of some "case studies" will illustrate to the reader... 

XPS [13, 14, 17-251 provides information about elemental surface compoaltion. The principle of phoioelectron spectroscopy is the cxcilation of electrons in an atom or molecule by meanj of X-rays into vacuum. The ejecied photoelectrons have a kineiic energy equal lo... 

The photoelectron effect, one of the basic principles of electron spectroscopy, was first explained by Albert Einstein in 1905, for which he received the 1921 Nobel Prize in Physics. 

X-ray photoelectron spectroscopy works on the principle of photoelectronic effect. The investigating surface is bombarded with X-ray photons, which leads to the emission of... 

Barr, T. L. (1994) Modern ESCA The Principles and Practice of X-ray Photoelectron Spectroscopy, CRC Press, Boca Raton, FL. 

Eland, J. H. D. (1983) Photoelectron Spectroscopy. 2nd edn, Butterworth-Heinemann, London. Huffier, S. (2001) Photoelectron Spectroscopy Principles and Applications. 3rd edn. Springer, Berlin. Prince, K. C. (1995) Photoelectron Spectroscopy of Solids and Suifaces Synchrotron Radiation Techniques and Applications, World Scientific Publishing, Singapore. 

Rabalais, J. W. (1977) Principles of Ultraviolet Photoelectron Spectroscopy, John Wiley, New York. Roberts, M. W. and McKee, C. S. (1979) Chemistry of the Metal-Gas Interface, Oxford University Press, Oxford. 

Several UHV techniques which have been developed have not found such wide use in corrosion analysis, despite potential applicability. Ultraviolet photoelectron spectroscopy (UPS) is one of these, operating in a similar fashion to XPS (but using an ultraviolet excitation), and probing the valence electrons, rather than the core electrons of the atoms. Because the energies of the valence electrons are so very sensitive to the precise state of the atom, the technique is in principle very informative however exactly this high sensitivity renders the data difficult to interpret, particularly as a routine... 

X-ray photoelectron spectroscopy (XPS), which is synonymous with ESCA (Electron Spectroscopy for Chemical Analysis), is one of the most powerful surface science techniques as it allows not only for qualitative and quantitative analysis of surfaces (more precisely of the top 3-5 monolayers at a surface) but also provides additional information on the chemical environment of species via the observed core level electron shifts. The basic principle is shown schematically in Fig. 5.34. 

Quantitative information about energies of atomic orbitals is obtained using photoelectron spectroscopy, which applies the principles of the photoelectric effect to gaseous atoms. Our Box (on the next page) explores this powerful spectroscopic technique. 

J. W. Rabalais, Principles of Ultraviolet Photoelectron Spectroscopy, Wiley, New York, 1977. 

Photoelectron spectroscopy (PES, a non-mass spectral technique) [87] has proven to be very useful in providing information not only about ionization potentials, but also about the electronic and vibrational structure of atoms and molecules. Energy resolutions reported from PES are in the order of 10-15 meV. The resolution of PES still prevents the observation of rotational transitions, [79] and to overcome these limitations, PES has been further improved. In brief, the principle of zero kinetic energy photoelectron spectroscopy (ZEKE-PES or just ZEKE, also a nonmass spectral technique) [89-91] is based on distinguishing excited ions from ground state ions. 

The theoretical interest in the LiH has increased since the electron affinity of LiH and its deuterated counterpart, LiD, were measured with the use of the photoelectron spectroscopy by Bowen and co-workers [126]. The adiabatic electron affinities of LiH and LiD determined in that experiment were 0.342 0.012 eV for the former and 0.337 0.012 eV for the latter system. The appearance of these data posed a challenge for theory to reproduce those values in rigorous calculations based on the first principles. Since the two systems are small, it has been particularly interesting to see if the experimental EAs can be reproduced in calculations where the BO approximation is not assumed [123]. 

The values of the ESP at the nuclear positions, as obtained from the electron and Hartree-Fock structure amplitudes for the mentioned crystals (using a K-model and corrected on self-potential) are given in table 2. An analysis shows that the experimental values of the ESP are near to the ab initio calculated values. However, both set of values in crystals differ from their analogs for the free atoms [5]. It was shown earlier (Schwarz M.E. Chem. Phys. Lett. 1970, 6, 631) that this difference in the electrostatic potentials in the nuclear positions correlates well with the binding energy of Is-electrons. So an ED-data in principle contains an information on the bonding in crystals, which is usually obtaining by photoelectron spectroscopy. 

As representative techniques of the second group, we discuss two methods x-ray photoelectron spectroscopy (XPS), sometimes referred to as electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES). The main principle of the first method (XPS) is the excitation of electrons in an atom or molecule by x-rays. The resulting electrons carry energy away according to the formula... 

Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) are the two principle surface analysis techniques. They are used to identify the elemental composition, i.e., the amount and nature of species present at the surface to a depth of about 1 nm. 

ESCA involves the measurement of binding energies of electrons ejected by interactions of a molecule with a monoenergetic beam of soft X-rays. For a variety of reasons the most commonly employed X-ray sources are Al and MgKol>2 with corresponding photon energies of 1486.6 eV and 1253.7 eV respectively. In principle all electrons, from the core to the valence levels can be studied and in this respect the technique differs from UV photoelectron spectroscopy (UPS) in which only the lower energy valence levels can be studied. The basic processes involved in ESCA are shown in Fig. 1. 

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