Diffusion, radiation induced

Even in the ab en(pe of preferential sputtering, i.e, SA=Sg, it is not necessarily the case that the concentration ratio of A and B is uniform with depth under steady state ion bombardment. The subsurface layer can have a different concentration ratio from the bulk or the top surface even if the latter two are identical. This can occur because of the subsurface effects of ion bombardment discussed below, which can produce an altered subsurface composition. As a result, there are two altered layers possible the top surface layer where composition at steady state is completely controlled by RgA and the subsurface layer where composition is controlled by segregation, radiation induced diffusion, radiation induced segregation, recoil implantation and cascade mixing. 

Different secondary ions can also display different depth resolution values for the same substrate. As an example. Copper typically yields poor depth resolution because of its high diffusion coefficient, particularly when sputtered. This sputter-induced enhancement is otherwise referred to as radiation-enhanced diffusion. Radiation-induced segregation may also be initiated, with different primary ion/secondary ion combinations resulting in different trends. As a result, any emissions collected as a result of this form of sputtering will always emanate from what is termed an altered layer, as opposed to the initial intrinsic substrate layer. Exceptions are sometimes noted for large cluster ion impact, because, as mentioned in Section 4.1.1.3, these can remove sputter-induced damage. 

The theory of radiation-induced grafting has received extensive treatment [21,131,132]. The typical steps involved in free-radical polymerization are also applicable to graft polymerization including initiation, propagation, and chain transfer [133]. However, the complicating role of diffusion prevents any simple correlation of individual rate constants to the overall reaction rates. Changes in temperamre, for example, increase the rate of monomer diffusion and monomer... 

The first of these factors reduces the complexity of the simulation, but the second has entirely the opposite effect as charge cycling events affect both the energy of the primary ion and its inelastic collision cross section. While the proximity of energy loss events does not affect the details of track structure simulation (at reasonable LET), it may cause significant complications in subsequent diffusion-kinetic calculations due to the (potentially unphysi-cally) high local concentration of radiation-induced reactants. 

It is known that more than 30 reactions are needed to reproduce the radiation-induced reactions occurring in pure water. Intensive measurements with a pulse radiolysis method have been done at elevated temperature up to 300°C [25 2], and the temperature dependence of some reactions does not exhibit a straight line but a curved one in Arrhenius plot. These examples are the reactions of the hydrated electron with N2O, NOJ, NO2, phenol, Se04, 8203 , and Mn [33,35], and two examples, egq + NOJ and ejq -i- NOJ, are shown in Fig. 2. The rate constant for the reaction of hydrated electron with NOJ is near diffusion-controlled reaction at room temperature and is increasing with increasing temperature. Above 100°C, the rate does not increase and reaches the maximum at 150°C, and then decreases. Therefore the curve is concave upward in Arrhenius plot. 

The dose rate affects both the yield and chain length of the grafted material. Air has a detrimental effect on grafting since it inhibits the reaction, which is consistent with other radiation-induced free radical reactions. Increasing the temperature of fhe graffing sysfem increases the yield. This is very likely because raising the temperature increases the diffusion rate of the monomer into the substrate. ... 

Effect of Sample Thickness. Sample thickness significantly affects the radiation-induced expansion of unstressed samples (Figure 14). The equilibrium asymptotic deflection values vary approximately as the square of the sample thickness for the 20- and 30-mil samples. The deflection values for all thicknesses above 0.010 follow the same curve at short times (—0.2 minute), where diffusion out of the sample has probably not yet become significant. 

Welsh suggested correctly that similar transitions take place even if the molecular pair is not bound. The energy of relative motion of the pair is a continuum. Its width is of the order of the thermal energy, Efree 3kT/2. Radiative transitions between free states occur (marked free-free in the figure) which are quite diffuse, reflecting the short lifetime of the supermolecule. In dense gases, such diffuse collision-induced transitions are often found at the various rotovibrational transition frequencies, or at sums or differences of these, even if these are dipole forbidden in the individual molecules. The dipole that interacts with the radiation field arises primarily by polarization of the collisional partner in the quadrupole field of one molecule the free-free and bound-bound transitions originate from the same basic induction mechanism. 

The authors of this book started working on chemical kinetics more than 10 years ago focusing on investigations of particular radiation - induced processes in solids and liquids. Condensed matter physics, however, treats point (radiation) defects as active particles whose individual characteristics define kinetics of possible processes and radiation properties of materials. A study of an ensemble of such particles (defects), especially if they are created in large concentrations under irradiation for a long time, has lead us to many-particle problems, common in statistical physics. However, the standard theory of diffusion-controlled reactions as developed by Smoluchowski... 

In the last several decades, both experimental data and theoretical studies [5, 9, 13-15] have revealed the effect of similar defect aggregation in the course of the bimolecular A+B —> 0 reaction under permanent particle source (irradiation) - the phenomenon similar to that discussed in previous Chapters for the diffusion-controlled concentration decay. Radiation-induced aggregation of similar defects being observed experimentally at 4 K after prolonged X-ray irradiation [16] via both anomalously high for random distribution concentration of dimer F2 centres (two nearest F centres) and directly in the electronic microscope [17], permits to accumulate defect concentrations whose saturation value exceed by several times that of the Poisson distribution. 

The current majority opinion is that both types of point defects are important. Thermal equilibrium concentrations of point defects at the melting point are orders of magnitude lower in Si than in metals. Therefore, a direct determination of their nature by Simmons-Balluffi-type experiments (26) has not been possible. The accuracy of calculated enthalpies of formation and migration is within 1 eV, and the calculations do not help in distinguishing between the dominance of vacancies or interstitials in diffusion. The interpretation of low-temperature experiments on the migration of irradiation-induced point defects is complicated by the occurrence of radiation-induced migration of self-interstitials (27, 28). 

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