Photoelectron emission process

The samples studied in connection with work discussed here consist of ultra-thin films of molecular solids, either polymers or condensed molecular solids. It is useful, however, to describe first the molecular photoelectron emission process, at least from a phenomenological view point, and then present the extra issues in dealing with solids. 

In a first approximation, we can consider the electron structure as frozen under the photoelectron emission process and identify Eb with the Hartree-Fock energy eigenvalues of the orbitals (Koopman s theorem). A schematic representation of an expected photoelectron spectrum could then be the one in Figure 9. 

In the photoelectron emission process, an incident photon of energy hw is absorbed by an atom. With that energy, a photoelectron is emitted with a kinetic energy equal to ... 

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). 

Wlien photons of sufiBciently high frequency v are directed onto a metal surface, electrons are emitted in a process known as photoelectron emission [ ]. The threshold frequency Vq is related to the work fimction by the expression... 

In X-ray photoelectron spectroscopy (XPS), a beam of soft X-rays with energy hv s. focused onto the surface of a solid that is held under an ultra-high vacuum, resulting in the ejection of photoelectrons from core levels of the atoms in the solid [20]. Fig. 15 shows an energy level diagram for an atom and illustrates the processes involved in X-ray-induced photoelectron emission from a solid. 

Figure 5.1 The X-ray emission and Auger processes (Pollard and Heron 1996 37). An inner shell vacancy is created in the K shell by the photoelectric process (emitted photoelectron not shown), (a) shows the X-ray emission process, where an L shell electron drops down to fill the vacancy, and the excess energy (EK - EL) is carried away as an X-ray photon. In (b), an L shell electron drops down, but the excess energy is carried away by an Auger electron emitted from the M shell, with kinetic energy approximately equal to EK - EL — EM. Reproduced by permission of the Royal Society of Chemistry.

Let us now briefly outline the structure of this review. The next section contains information concerning the fundamentals of the electrochemistry of semiconductors. Part III considers the theory of processes based on the effect of photoexcitation of the electron ensemble in a semiconductor, and Parts IV and V deal with the phenomena of photocorrosion and light-sensitive etching caused by those processes. Photoexcitation of reactants in a solution and the related photosensitization of semiconductors are the subjects of Part VI. Finally, Part VII considers in brief some important photoelectrochemical phenomena, such as photoelectron emission, electrogenerated luminescence, and electroreflection. Thus, our main objective is to reveal various photo-electrochemical effects occurring in semiconductors and to establish relationships among them. 

Photoelectrochemistry (PEC) is emerging from the research laboratories with the promise of significant practical applications. One application of PEC systems is the conversion and storage of solar energy. Chapter 4 reviews the main principles of the theory of PEC processes at semiconductor electrodes and discusses the most important experimental results of interactions at an illuminated semiconductor-electrolyte interface. In addition to the fundamentals of electrochemistry and photoexcitation of semiconductors, the phenomena of photocorrosion and photoetching are discussed. Other PEC phenomena treated are photoelectron emission, electrogenerated luminescence, and electroreflection. Relationships among the various PEC effects are established. 

Fig. 1. Comparison of the four different physical processes which can be observed during the interaction of X-ray photons with matter 2 1. The two phenomena scetched below, namely photoelectron emission and Auger electron emission, can be detected and measured in a photoelectron spectrometer by determining the kinetic energy of the ejected free electrons...

Hitherto the discussion of Fig. 5.2 has neglected the possibility of non-radiative decay following 4d shell excitation/ionization. These processes are explained with the help of Fig. 5.2(h) which also reproduces the photoelectron emission discussed above, because both photo- and autoionization/Auger electrons will finally yield the observed pattern of electron emission. (In this context it should be noted that in general such direct photoionization and non-radiative decay processes will interfere (see below).) As can be inferred from Fig. 5.2(h), two distinct features arise from non-radiative decay of 4d excitation/ionization. First, 4d -> n/ resonance excitation, indicated on the photon energy scale on the left-hand side, populates certain outer-shell satellites, the so-called resonance Auger transitions (see below), via autoionization decay. An example of special interest in the present context is given by... 

There are other electron emission processes which lie between such well-defined limiting cases, e.g., resonance affected two-electron emission which lies between direct double photoionization and photon-induced two-step double ionization (photoelectron and Auger electron emission). 

The electron affinity can also be deduced from the measurement of the spectrum of the photoelectron emission with monochromatic UV light. This technique is ultra-violet (UV) photoelectron emission spectroscopy (or UV photoemission spectroscopy or UPS). The UPS technique involves directing monochromatic UV light to the sample to excite electrons from the valence band into the conduction band of the semiconductor. Since the process occurs near the surface, electrons excited above the vacuum level can be emitted into vacuum. The energy analysis of the photoemitted electrons is the photoemission spectrum. The process is often described in terms of a three step model [8], The first step is the photoexcitation of the valence band electrons into the conduction band, the second step is the transmission to the surface and the third step is the electron emission at the surface. The technique of UPS is probably most often employed to examine the electronic states near the valence band minimum. 

Table 4 contains a collection of diffusion coefficients determined experimentally for a variety of adsorbate systems. It shows that the values may vary considerably, which is of course due to the specific bonding of the adsorbate to the surface under consideration. Surface diffusion plays a vital role in surface chemical reactions because it is one factor that determines the rates of the reactions. Those reactions with diffusion as the rate-determining step are called diffusion-limited reactions. The above-mentioned photoelectron emission microscope is an interesting tool to effectively study diffusion processes under reaction conditions [158], In the world of real catalysts, diffusion may be vital because the porous structure of the catalyst particle may impose stringent conditions on molecular diffusivities, which in turn leads to massive consequences for reaction yields. 

A variety of processes may take place at this stage, distinguishing the different techniques — photoelectron emission, Auger electron emission, ion emission, energy loss due to core-hole, plasmon or phonon excitation, as well as... 

From the point of view of obtaining surface structure information, the nature of this primary process is important in two ways. First, the process may be atom-specific, giving chemical selectivity, as in photoelectron emission and Auger electron emission. Second, the process can affect the characteristics of the outgoing particles, especially in terms of angular distribution, and in terms of the exact location within the surface where this primary process takes place. The angular distribution can give direct evidence of the molecular orientation with respect to the surface this happens, for instance, when an electron is emitted from a valence orbital. 

In terms of location, the primary process may be concentrated at one atom, as in an electron-atom scattering in LEED or in electron emission from core levels. It may also be delocalized over many atoms, as in photoelectron emission from a delocalized valence band level. The latter case is again of value to obtain molecular orientations directly. The polarization of the incident electron may also be used to determine molecular orientations, through its effect on the primary process. 

In section 2.3 we have decomposed various surface-sensitive techniques into a small set of more elementary processes. In this section we shall present appropriate formalisms that describe these individual processes. Namely, we shall discuss the theoretical treatment of the propagation of electrons in the surface region, as well as the treatment of elastic and inelastic electron-atom scattering, photoelectron emission and of Auger electron emission. These parts... 

Figure 7.1 Emission processes of characteristic electrons (a) a Is photoelectron and (b) a KL1L2,3 Auger electron...

Absorption means diminution of coherent x-ray intensity in the crystal through inelastic processes such as atomic absorption and fluorescence, photoelectron emission, and Compton effect extinction means intensity diminution due to loss through diffraction by fortuitously oriented mosaic blocks. The simple extinction expression due to Darwin, given in Eq. (18), is only a rough approximation more accurate treatments will be mentioned in what follows. In Eq. (17) the absorption factor is expressed in terms of the linear absorption coefficient /inn (calculated from tabulated values of the elemental atomic or mass absorption coefficients, updated values of which will appear in Vol. IV of International Tables,2 the path length f, of the incident ray from the crystal surface to the point of diffraction r, and the path length t2 of the diffracted ray from that point to the crystal surface. 

Apparently, in the near future there will be developed (a) a detailed theory of surface excitons not only at the crystal boundary with vacuum but also at the interfaces of various condensed media, particularly of different symmetry (b) a theory of surface excitons including the exciton-phonon interaction and, in particular, the theory of self-trapping of surface excitons (c) the features of surface excitons for quasi-one-dimensional and quasi-two-dimensional crystals (d) the theory of kinetic parameters and, particularly, the theory of diffusion of surface excitons (e) the theory of surface excitons in disordered media (f) the features of Anderson localization on a surface (g) the theory of the interaction of surface excitons of various types with charged and neutral particles (h) the evaluation of the role of surface excitons in the process of photoelectron emission (i) the electronic and structural phase transitions on the surface with participation of surface excitons. We mention here also the theory of exciton-exciton interactions at the surface, the surface biexcitons, and the role of defects (see, as example, (53)). The above list of problems reflects mainly the interests of the author and thus is far from complete. Referring in one or another way to surface excitons we enter into a large, interesting, and yet insufficiently studied field of solid-state physics. 

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