Photoelectron spectroscopy primary processes

The fundamental physical processes occurring in photoelectron spectroscopy are given in Table 9.1. The primary process is an ionization step induced by a photon. Photoionization produces an electron with discrete kinetic energy. The kinetic energy of the electron, KE is given by... 

Figure 9 Anion photoelectron spectroscopy. Its unique features are (I) Intrinsic mass selectivity and (ii) neutrals as final states. Here, as an example the results for compounds of iron, carbon and hydrogen are shown which exist in catalytic processes, high-temperature terrestrial or low-temperature astrophysical chemistry. Bottom spectrum a primary anion mass spectrum containing anions of the complexes of interest. Top spectra anion photoelectron spectra obtained by electron kinetic energy analysis after laser-induced photodetachment. They reveal the change of molecular structure and electronic energies for increasing numbers of hydrogen atoms in the complex.

In the case of ultraviolet photoelectron spectroscopy, photons in the energy range of up to 100 eV are used for the primary excitation process. As a result, information on core-level binding energies cannot be obtained and the type of elements present on a surface cannot easily be determined. The photoelectrons emitted from the sample upon absorption of an UV-photon originate from more weakly bound electronic... 

X-ray photoelectron spectroscopy (XPS) operates on the principle of the photoelectric effect, which occurs via a primary excitation process brought about by X-ray-irradiation producing electrons photoelectrons) of discrete energy, containing chemical information regarding the surface analyte. It should be noted that X-rays are only one of many types of excitation sources that can be used to induce emission of electrons for analysis. X-ray photoelectron (XP) spectral peaks (generated by the photoelectrons) are named according to the orbital 1 = 0, 1,2,3... denoted as s, p, d, f...) and spin s = 1/2) quantum numbers of the core levels from which they emanate. The total momentum of the photoelectrons ( / = / x) is included... 

In order to appreciate these and other results of yield spectroscopy on NEA diamond surfaces, it is best to recall briefly Spicer s three-step model of photoelectron emission, which is likely to be nowhere better suited than in the case at hand [109]. This model divides the photoelectron emission process up into three conceptually separate processes, (i) The bulk absorption of light generates photoexcited electrons and holes, and (ii) electrons travel to the surface with the possibility to suffer inelastic losses on their way before they (iii) escape into vacuum where they are being detected. In normal photoelectron spectroscopy interest lies in the so-called primary current, that is, in those electrons that leave the sample without energy loss on their way to the surface. In this case, the photoexcitation, transport, and escape processes are not entirely independent. For crystalline samples with well-ordered surfaces, the wave vector component parallel to the surface, k, is, for example, conserved from the initial electron state to the free electron in vacuum. In this case, a better description of the photoelectron emission is by a one-step excitation from an initial band structure state to a final state constructed as an inverse LEED state (Chapter 3.2.2). The inelastic mean free path of photoexcited electrons, is energy dependent and lies in the nanometer range (Chapter 3.2.3). 

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