Ionization techniques
Surface composition can be determined by a variety of ionization techniques. The same techniques can be used to detect any remaining contamination after cleaning and to detect layers of material adsorbed later in the experiment. Their common feature is that the escape depth of the electrons, the maximum depth from which ejected electrons come, is in the range 0.1–1.0 nm, which ensures that only surface species contribute. One technique that may be used is photoelectron spectroscopy (Section 11.4), which in surface studies is normally called photoemission spectroscopy. X-rays or hard ultraviolet ionizing radiation of energy in the range 5–40 eV may be used, giving rise to the techniques denoted XPS and UPS, respectively. In XPS, the energy of the incident photon is so great that electrons are ejected from inner cores of atoms. As a first approximation, core ionization energies are insensitive to the bonds between atoms because they are too tightly bound to be greatly affected by the changes that accompany bond formation, so core ionization energies are char acteristic of the individual atom. Consequently, XPS gives lines characteristic of the elements present on a surface. For instance, the K-shell ionization energies of the second row elements are
Li Be B C N O F
50 110 190 280 400 530 690ev
Detection of one of these values (and values corresponding to ejection from other inner shells) indicates the presence of the corresponding element (Fig. 25.8). This application is responsible for the alternative name electron spectroscopy for chemical analysis (ESCA). The technique is very useful for studying the surface state of heterogeneous catalysts, the differences between surface and bulk structures, and the processes that can cause damage to high-temperature superconductors and semi conductor wafers. UPS, which examines electrons ejected from valence shells, is more suited to establishing the bonding characteristics and the details of valence shell electronic structures of substances on the surface. Its usefulness is its ability to reveal which orbitals of the adsorbate are involved in the bond to the substrate. For instance, the principal difference between the photoemission results on free benzene and benzene adsorbed on palladium is in the energies of the π electrons. This difference is interpreted as meaning that the C6H6 molecules lie parallel to the surface and are attached to it by their π orbitals. In secondary-ion mass spectrometry (SIMS), the surface is ionized by bombard ment with other ions and the secondary ions that emerge from the surface are detected by a mass spectrometer. Among the advantages of SIMS are the ability to detect adsorbed H and He atoms, which are not easily probed by XPS, and the high sensitivity of the mass spectrometer detector. A disadvantage is that SIMS analysis erodes the part of the sample that is bombarded. However, it is possible to control the degree of erosion to one or two monolayers by controlling the bombardment parameters.
Fig. 25.8 The X-ray photoelectron emission spectrum of a sample of gold contaminated with a surface layer of mercury. (M.W. Roberts and C.S. McKee, Chemistry of the metal–gas interface, Oxford (1978).)
