Photoemission surface sensitivity

A number of surface-sensitive spectroscopies rely only in part on photons. On the one hand, there are teclmiques where the sample is excited by electromagnetic radiation but where other particles ejected from the sample are used for the characterization of the surface (photons in electrons, ions or neutral atoms or moieties out). These include photoelectron spectroscopies (both x-ray- and UV-based) [89, 9Q and 91], photon stimulated desorption [92], and others. At the other end, a number of methods are based on a particles-in/photons-out set-up. These include inverse photoemission and ion- and electron-stimulated fluorescence [93, M]- All tirese teclmiques are discussed elsewhere in tliis encyclopaedia. 

Depending on the energy tico of the incident photons, valence band states and even core level electrons can be excited. UPS is a surface-sensitive technique since electrons have a very short inelastic mean free path, Xi, which depends on the kinetic energy Ek, and has a minimum value of 0.5 nm for T k 100 eV. The leading edge of the valence band is taken as the VBM or HOMO maximum and has to be referred to which has to be determined from a clean inorganic metal surface. Those electrons with k > 0 are removed from the sample and transmitted to the detector. The fundamental equation of the photoemission process is (Einstein, 1905) ... 

Surface-induced effects can be expected to distort considerably more the spectra measured for excitation energies of 40.8 and 48.4 eV than for an excitation energy of 21.2 eV (and also 1253.6 eV) since surface sensitivity is higher for these exciting energies. Two different surface effects can contribute to the photoemission spectrum ... 

Four UHV spectroscopies used for the compositional and chemical analysis of surfaces are discussed. These are X-ray Photoemission, Auger Spectroscopy, Secondary Ion Mass Spectroscopy, and Ion Scattering (both low and high energy). Descriptions of the basic processes and information contents are given, followed by a comparative discussion of the surface sensitivities, advantages and disadvantages of each spectroscopy. 

Fine structure experiments are often carried out with synchrotron sources, since the initial electron state is better defined for photoemission than for electron excitation. When core-hole decay is detected by Auger or secondary electron emission, the technique is surface sensitive. Core-hole decay can also be detected by fluorescence, or by adsorption of the incident photon beam. These methods are not intrinsically surface sensitive, but they are useful when the source atoms are exclusively located at the surface. 

Photoemission spectroscopy involves measurement of the energy distribution of electrons emitted from a solid under irradiation with mono-energetic photons. In-house experiments are usually performed with He gas discharge lamps which generate vacuum UV photons at 21.2 eV (He la radiation) or 40.8 eV (He Ila radiation ) or with Mg Ka (hv=1284.6 eV) or A1 Ka (hv=1486.6eV) soft X-ray sources. UV photoemission is restricted to the study of valence and conduction band states, but XPS allows in addition the study of core levels. Alternatively photoemission experiments may be performed at national synchrotron radiation facilities. With suitable choice of monochromators it is possible to cover the complete photon energy range from about 5 eV upward to in excess of 1000 eV. The surface sensitivity of photoemission derives from the relatively short inelastic mean free path of electrons in solids, which reaches a minimum of about 5A for electron energies of the order 50-100 eV. 

A photomultiplier (PM) tube is more sensitive than a phototube for the visible and ultraviolet regions. It consists of a photoemissive cathode, which the photon strikes, and a series of electrodes (dynodes), each at a more positive potential (50 to 90 V) than the one before it. When an electron strikes the photo-emissive surface, a primary electron is emitted (this is the photoelectric effect— Albert Einstein received the 1921 Nobel Prize in Physics for its discovery in 1905, not for the special theory of relativity which he also introduced in 1905—see www.lucidcafe.com/lucidcafe/librarv/96mar/einstein.htmD. The primary electron released from the photoemissive surface is accelerated toward the first dynode. The impact of the electron on the dynode surface causes the release of many secondary electrons, which in turn are accelerated to the next electrode where each secondary electron releases more electrons, and so on, up to about 10 stages of amplication. The electrons are finally collected by the anode. The final output of the photomultiplier tube may, in turn, be electronically amplified. 

Surface sensitivity is an intrinsic property of photoemission measurements. The incident light penetrates far into the solid, but the escape depth of excited electrons is very short (Fig. 5), although there are local variations related to direction-dependent band structure effects. Surface sensitivity can be further enhanced by appropriate choice of experimental parameters such as photon energy, angles of incidence and emission, etc., which take advantage of selection rules favouring surface processes. 

Information on the spin resolved band structure of ferromagnetic materials can directly be obtained from spin resolving photoelectron spectroscopy. Using polarized radiation spin integrating photoemission techniques already enable to have access to magnetic properties. An enhancement of the surface sensitivity can be achieved using neutral excited spin polarized atoms which move towards the sample and are de-excited by tunneling electrons from the surface with a subsequent emission of electrons. 

Apparently in contradiction the photoemission spectra (see Fig. 5.3) exhibit a dominance of majority electrons near the Fermi level. However, keeping in mind the calculation of Wu et al. [50] one can now easily realize the distinct surface sensitivity of metastable de-excitation spectroscopy (MDS) which gives predominantly information from the topmost surface layer whereas in photoemission experiments the information depth is a few layers. 

Figure 2.51 X-ray photoelectron valence band spectrum at high surface sensitivity (hv= ISOeV) showing the energetic difference between Fermi level and valence band photoemission onset to be 1.06eV.

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