Knock-on electrons

The distribution of energetic knock-on electrons (8 rays) with kinetic energies lis given by Rossi (1952) as... 

The total energy loss can be related to the number of ion pairs produced near the particle track. This relation becomes complicated for relativistic (/ approaches to 1) particles due to the escape of energetic knock-on electrons whose range may exceed the dimensions of the sample. The mean local energy dissipation per ion pair produced, W, is essentially constant for relativistic particles and slowly increases at lower energies (Bichsel et al. 2002). It can be sensitive to the presence of contaminants in gases and may be influenced by subsequent recombination. The ionization yields are very important for a number of applications, especially for radiation detectors. 

Radiation Damage. It has been known for many years that bombardment of a crystal with energetic (keV to MeV) heavy ions produces regions of lattice disorder. An implanted ion entering a soHd with an initial kinetic energy of 100 keV comes to rest in the time scale of about 10 due to both electronic and nuclear coUisions. As an ion slows down and comes to rest in a crystal, it makes a number of coUisions with the lattice atoms. In these coUisions, sufficient energy may be transferred from the ion to displace an atom from its lattice site. Lattice atoms which are displaced by an incident ion are caUed primary knock-on atoms (PKA). A PKA can in turn displace other atoms, secondary knock-ons, etc. This process creates a cascade of atomic coUisions and is coUectively referred to as the coUision, or displacement, cascade. The disorder can be directiy observed by techniques sensitive to lattice stmcture, such as electron-transmission microscopy, MeV-particle channeling, and electron diffraction. 

For knock-on collisions, one uses the Rutherford cross section for free electrons, and the number of free electrons is taken equal to the integral of the oscillator strength up to the energy loss e (dispersion approximation). Thus,... 

To calculate the energy partition between the core and the envelope, Mozumder et al. (1968) considered the equipartition of deposited energy between glancing and knock-on collisions (Sect. 2.3.4). Of the ejected electrons... 

If a surface reaction is to involve more than monolayer-chemisorption, then the species adsorbed on the surface must be able to migrate into the second and deeper layers forming new chemical bonds and often new molecular species. This is step 3, product formation, and it often requires an activation mechanism to proceed, i.e., a monolayer is formed and the reaction stops unless the substrate is held at elevated temperature or there is ion or electron bombardment. Damage-enhanced diffsusion, knock-on collisions, and bond breaking may promote the reaction in the presence of ion bombardment. Although the precise mechanisms are unclear, it is certain that electron and ion bombardment cause step 3 to occur in some instances where the chemical reaction does not proceed in the absence of radiation. 

Compared with the momentum of impinging atoms or ions, we may safely neglect the momentum transferred by the absorbed photons and thus we can neglect direct knock-on effects in photochemistry. The strong interaction between photons and the electronic system of the crystal leads to an excitation of the electrons by photon absorption as the primary effect. This excitation causes either the formation of a localized exciton or an (e +h ) defect pair. Non-localized electron defects can be described by planar waves which may be scattered, trapped, etc. Their behavior has been explained with the electron theory of solids [A.H. Wilson (1953)]. Electrons which are trapped by their interaction with impurities or which are self-trapped by interaction with phonons may be localized for a long time (in terms of the reciprocal Debye frequency) before they leave their potential minimum in a hopping type of process activated by thermal fluctuations. 

The second subregion corresponds to large to and q and is related to close collisions (the knock-on). At very large transferred momenta (qa0> 1) the inelastic scattering by a molecule (an atom) is actually the elastic scattering by a free electron with the cross section given by Rutherford formula. In this case the function /(to, q) can be presented analytically as a delta function ... 

A heavy charged particle can knock out an electron from a molecule with maximum energy Emax —2/ra>2 (at v>v0), whereas for a fast electron with the same velocity the knocked out electron has max — mu2/2. Consequently, while an electron can knock out electrons with velocity no greater than its own, a heavy particle, in head-on collisions, produces delta electrons with velocities which can be twice as high as that of the ion. As a result, the energy of such delta electrons can be distributed to the regions of the medium far more remote from the point of initial ionization than in the case of electron irradiation. 

Further evidence for the importance of electronic excitation as a primary means of defect creation comes from studies of "sub-threshold" damage in which the energy of the incoming particle is less than that required for a "knock-on" collision. 

Mass spectrometry is used to measure the molecular mass of a compound and provides a method to obtain the molecular formula. It differs from the other instrumental techniques presented thus far because it does not involve the interaction of electromagnetic radiation with the compound. Instead, molecules of the compound being studied are bombarded with a high-energy beam of electrons in the vapor phase. When an electron from the beam impacts on a molecule of the sample, it knocks an electron out of the molecule. The product, called the molecular ion (represented as A/f), has the same mass as the original molecule but has one less electron. It has both an odd number of... 

With carbonization, coloration of the polymers occurs [21]. Mechanisms of coloration or blackening of polymers induced by ion beams have been studied [22, 23] and two different models of blackening processes have been proposed direct knock-on of atoms from polymer chains by nuclear collision [22] and high density electronic excitation effects by an electronic excitation process [23]. 

Whenever light produces an observable effect, for example, when it acts on a photographic plate or knocks an electron out of an atom, it appears to act like particles. In interference experiments it is not the waves which are observed, but the distribution of light intensity. This is done by means of a photographic plate, or in some other equivalent way. The observed distribution of intensity is thus not a distribution of waves but a distribution of photons, or rather a distribution of effects attributed to photons. The photons themselves are not observed any more than the waves are. 

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