Electromagnetic radiation photoelectron spectroscopy

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. 

Most of what we know about the structure of atoms and molecules has been obtained by studying the interaction of electromagnetic radiation with matter. Line spectra reveal the existence of shells of different energy where electrons are held in atoms. From the study of molecules by means of infrared spectroscopy we obtain information about vibrational and rotational states of molecules. The types of bonds present, the geometry of the molecule, and even bond lengths may be determined in specific cases. The spectroscopic technique known as photoelectron spectroscopy (PES) has been of enormous importance in determining how electrons are bound in molecules. This technique provides direct information on the energies of molecular orbitals in molecules. 

Spectroscopy produces spectra which arise as a result of interaction of electromagnetic radiation with matter. The type of interaction (electronic or nuclear transition, molecular vibration or electron loss) depends upon the wavelength of the radiation (Tab. 7.1). The most widely applied techniques are infrared (IR), Mossbauer, ultraviolet-visible (UV-Vis), and in recent years, various forms ofX-ray absorption fine structure (XAFS) spectroscopy which probe the local structure of the elements. Less widely used techniques are Raman spectroscopy. X-ray photoelectron spectroscopy (XPS), secondary ion imaging mass spectroscopy (SIMS), Auger electron spectroscopy (AES), electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopy. 

Most other forms of spectroscopy do not involve emission of extra particles such as electrons, but the straightforward absorption or emission of photons. These processes increase or decrease the energy of an atom ex molecule, by an amount equal to the photon energy. The results all reinforce the conclusion of photoelectron spectroscopy that only discrete energy levels occur (see Fig. 1.12). For example, the line spectra of atoms, known since the early nineteenth century, only contain lines at certain well-defined wavelengths. The quantization of energy, not only in electromagnetic radiation but in material systems, is an inescapable conclusion rtf spectroscopy. 

If electromagnetic radiation with a sufficiently short wavelength hits a specimen, then electrons may be excited and leave its surface. By the use of X-rays, also electrons of core levels may be emitted. X-ray Photoelectron Spectroscopy (XPS) examines the kinetic energy of these photo-emitted electrons in an electrostatic energy analyzer. A simple energy balance permits the evaluation of the binding energy Eg of the level from which the electron is emitted. 

The technique of photoelectron spectroscopy (PES, also called photoemission spectroscopy), was developed in the 1960s independently by Turner (in Oxford), Spicer (in Stanford), Vilesov (in Leningrad, now St. Petersburg) and Siegbahn (in Uppsala). In a PES experiment, atoms or molecules are excited with monochromated electromagnetic radiation of energy, causing electrons to be ejected, i.e. photoionization occurs (eq. 4.24). 

In vacuum ultraviolet photoelectron spectroscopy (UPS), the sample atom or molecule is exposed to radiation in the vacuum ultraviolet region of the electromagnetic spectrum. A readily available source of radiation is the helium discharge lamp, whieh produces a sharp Hel line at 21.2 eV. Since the energy required for photoionization of sets of valence electrons is in the vicinity of 6 eV to this energy, we obtain a polyenergetic emission of electrons described by the Einstein relation... 

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