Synchrotron XPS

Neilson GW, Enderby JE (1989) The coordination of metal aqua ions. Adv Inorg Chem 34 195-218 Neilson GW, Enderby JE (1996) Aqueous solutions and neutron scattering. J Phys Chem 100 1317-1322 Nesbitt HW, Scaini M, Hochst H, Bancroft GM, Schaufuss AG, Szargan R (2000) Synchrotron XPS evidence for Fe2+-S and Fe3+-S surface species on pyrite fracture-surfaces, and their 3D electronic states. Am Mineral 85 850-857... 

Fig. 10 Synchrotron XPS S 2p spectrum of pure chalcopyrite a) vacuum fractured surface, b) oxidized for 30 min at pH 9, c) leached for 2h in pH 1. Adapted from Ref. 117 with permission from Elsevier.

XPS and Synchrotron XPS at the sulfur edge have been used for investigating the sulfur atoms on chalcopyrite surface. Figure 10 shows synchrotron XPS sulfur 2p core-line spectra of a pristine vacuum fractured mineral and oxidized surfaces at pH 9 and The doublet in the spectra shown in Fig. 10a is a characteristic feature for pyrite, chalcopyrite and molybdenite, and it occurs due to the spin orbit coupling on sulfur. The fit... 

No commercial instruments specifically for UPS usually an add-on to XPS (incremental cost " 30,000) or done using a synchrotron facility as the photon source... 

Unlike traditional surface science techniques (e.g., XPS, AES, and SIMS), EXAFS experiments do not routinely require ultrahigh vacuum equipment or electron- and ion-beam sources. Ultrahigh vacuum treatments and particle bombardment may alter the properties of the material under investigation. This is particularly important for accurate valence state determinations of transition metal elements that are susceptible to electron- and ion-beam reactions. Nevertheless, it is always more convenient to conduct experiments in one s own laboratory than at a Synchrotron radiation focility, which is therefore a significant drawback to the EXAFS technique. These focilities seldom provide timely access to beam lines for experimentation of a proprietary nature, and the logistical problems can be overwhelming. 

As stated earlier, the major use of UPS is not for materials analysis purposes but for electronic structure studies. There are analysis capabilities, however. We will consider these in two parts those involving the electron valence energy levels and those involving low-lying core levels accessible to UPS photon energies (including synchrotron sources). Then we will answer the question why use UPS if XPS is available ... 

Table 1 lists core levels and their BEs for elements commonly used in technology, which are sufficiendy sharp and intense, and which are accessible to laboratory He I or He II sources (21.2-eV or 40.8-eV photon energy) or to synchrotron sources (up to 200 eV or higher). The analytical approaches are the same as described in the XPS article. For example, in that article examples were given of Si 2p spectra obtained using a laboratory A1 Ka X-ray source at l486-eV photon energy. The... 

Si 2p line, at about 100 eV BE, is also easily accessible at most synchrotron sources but cannot, of course, be observed using He I and He II radiation. On the other hand, the Zn 3d and Hg 4f lines can be observed quite readily by He I radiation (see Table 1) and the elements identified in this way. Quantitative analysis using relative peak intensities is performed exactly as in XPS, but the photoionization cross sections a are very different at UPS photon energies, compared to A1 Ka energies, and tabulated or calculated values are not so readily available. Quantitation, therefore, usually has to be done using local standards. 

Secondly, spectral resolution can be significantly higher for UPS or synchrotron data, compared to XPS. This is simply a consequence of UPS (synchrotron) sources... 

The discrete line sources described above for XPS are perfectly adequate for most applications, but some types of analysis require that the source be tunable (i.e. that the exciting energy be variable). The reason is to enable the photoionization cross-section of the core levels of a particular element or group of elements to be varied, which is particularly useful when dealing with multielement semiconductors. Tunable radiation can be obtained from a synchrotron. 

In a synchrotron, electrons are accelerated to near relativistic velocities and constrained magnetically into circular paths. When a charged particle is accelerated, it emits radiation, and when the near-relativistic electrons are forced into curved paths they emit photons over a continuous spectrum. The general shape of the spectrum is shown in Fig. 2.4. For a synchrotron with an energy of several gigaelectronvolts and a radius of some tens of meters, the energy of the emitted photons near the maximum is of the order of 1 keV (i.e., ideal for XPS). As can be seen from the universal curve, plenty of usable intensity exists down into the UV region. With suitable mono-... 

Tonner et al. have taken scanning XPS microscopies at the Advanced Light Source Synchrotron Radiation Center of Lawrence Berkeley National Laboratory [2.6]. They investigated a polished and sputter-cleaned surface of mineral ilmenite with the nominal composition FeTi03, and used the Fe 3p and Ti 3p lines for imaging. Using synchrotron radiation they demonstrated spatial resolution of approximately 0.25 p,m. 

Other techniques utilize various types of radiation for the investigation of polymer surfaces (Fig. 2). X-ray photoelectron spectroscopy (XPS) has been known in surface analysis for approximately 23 years and is widely applied for the analysis of the chemical composition of polymer surfaces. It is more commonly referred to as electron spectroscopy for chemical analysis (ESCA) [22]. It is a very widespread technique for surface analysis since a wide range of information can be obtained. The surface is exposed to monochromatic X-rays from e.g. a rotating anode generator or a synchrotron source and the energy spectrum of electrons emitted... 

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