Electron cascades

As an example of the use of AES to obtain chemical, as well as elemental, information, the depth profiling of a nitrided silicon dioxide layer on a silicon substrate is shown in Figure 6. Using the linearized secondary electron cascade background subtraction technique and peak fitting of chemical line shape standards, the chemistry in the depth profile of the nitrided silicon dioxide layer was determined and is shown in Figure 6. This profile includes information on the percentage of the Si atoms that are bound in each of the chemistries present as a function of the depth in the film. 

The time-resolved aspect of domino processes would, however, be in agreement with cascade reactions as a third expression used for the discussed transformations. Unfortunately, the term cascade is employed in so many dilferent connections - for example, photochemical cascades, biochemical cascades or electronic cascades - on each occasion aiming at a completely dilferent aspect, that it is not appropriate moreover, it also makes the database search much more difficult Moreover, if water molecules are examined as they cascade, they are simply moving and do not change. Several additional excellent reviews on domino reactions and related topics have been published [7], to which the reader is referred. 

Dynode strings can be constructed in many ways and the response time and range of linearity of the detector depend on the configuration. In the Venetian blind configuration (Fig. 2.21c) the dynodes are wide strips of material placed at an angle of 45° with respect to the electron cascade axis. This system offers a large input area to the incident primary particles. The advantage is that the dynodes are easily placed in line and the dimensions... 

Electron capture, in which an inner-shell electron is captured by a proton in the nucleus with the formation of a neutron. X-rays are emitted as the electrons cascade down to fill the vacancy in the lower energy level. 

WolfE and more recently other investigators have applied the Boltzmann diffusion equation to a description of the secondary-electron cascade. This approach is quite satisfying because it has a clearly defined foundation which seems to encompass all of the basic physical processes needed to describe the situation. It also yields an approximate solution in analytic form which is given by Equation 22. 

Besides space charge, image quality is limited by the combination of the number of pixels of the CCD camera and the size of a single ion event. The PSD usually employs two micro-channel plates (MCPs) mounted in a chevron configuration and backed by a phosphor screen. A product ion causes an electron cascade through the channel plates and onto the phosphor, which emits a pulse of diffuse light that must be focused properly onto the CCD array with a camera lens or fiber optic taper. In this process one product ion results in a signal on the CCD that is usually several pixels... 

Figure 7-12 Discrete dynode electron multiplier showing dynode structure and generation of electron cascade.

Energetically, the core hole is filled by an electron cascading from a higher energy orbital along with the simultaneous ejection of yet another electron. The process is a simultaneous two-electron coulombic readjustment by the remaining electrons to the core hole. 

A further system providing photoswitchable redox-activated properties with amplification features via a secondary electrocatalytic vectorial electron transfer reaction has been exemplified by diarylethene (45) molecules incorporated into a long-chain thiol monolayer adsorbed on a Au electrode due to hydrophobic interactions [85]. In the closed isomeric state (45a), the monolayer demonstrates well-defined reversible cyclic voltammetry, whereas the open (45b)-state is completely redox-inactive. The electrochemically active 45a-state provides electrocatalytic reduction of Fe(CN)g-, thus enabling a vectorial electron cascade that amplifies the photonic input. 

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