Elementary atomic processes

The elementary atomic processes governing the growth of nanostructures on surfaces under ultrahigh vacuum (UHV) conditions are summarized in Fig. 8. They include diffusion processes on terraces, at edges, between layers and across steps, as well as nucleation and coalescence. Each of these processes i is temperature dependent and can be described by a simple Arrhenius-type kinetic equation (12) ... 

Nikitin E E 1974 Theory of Elementary Atomic and Molecular Processes In Gases (Qxford Ciarendon)... 

Fast transient studies are largely focused on elementary kinetic processes in atoms and molecules, i.e., on unimolecular and bimolecular reactions with first and second order kinetics, respectively (although confonnational heterogeneity in macromolecules may lead to the observation of more complicated unimolecular kinetics). Examples of fast thennally activated unimolecular processes include dissociation reactions in molecules as simple as diatomics, and isomerization and tautomerization reactions in polyatomic molecules. A very rough estimate of the minimum time scale required for an elementary unimolecular reaction may be obtained from the Arrhenius expression for the reaction rate constant, k = A. The quantity /cg T//i from transition state theory provides... 

It is recalled that the elementary atomic migration by breaking bondings with surrounding atoms is also driven by thermal activation process. This is modeled through the incorporation of the activation barrier, AG, in the spin flipping event via the following equation. 

The results of these experiments also confirm the conclusions made in the above paper dealing with the mechanism of aggregation. These conditions were based on the data obtained using the method of semiconductor sensors. However, the technique used in [42] was seemingly more sensitive, because it enabled observation of elementary surface processes, such as the appearance of centres of condensation of metal atoms on atomic scale. 

E.E. Nikitin, Theory of Elementary Atomic-Molecular Processes in Gases, Khimiya, Moscow, 1970. 

The more recent theories of chemical conversions [59-61] take into account the fact that the process of overcoming the activation barrier involves a cooperative change of more than one degree of freedom for the starting reagents subsystem. For the surface processes this is expected to lead to a need for considering the dynamics of the solid atom motion and, at least, the model should include information on Debye frequencies for its atoms (see, e.g., Ref. [62]). An additional inconvenience of the models for the elementary surface processes is associated with the fact that the frequencies of the surface atom oscillations differ from those inside the solid. Consideration of the multiphonon contributions to the probabilities that the elementary process can take place results in a significant modification of its rate constant up to the complete disappearance of the activation form of the temperature dependence [63,64]. 

The atomic processes that are occurring (under conditions of equilibrium or non equilibrium) may be described by statistical mechanics. Since we are assuming gaseous- or liquid-phase reactions, collision theory applies. In other words, the molecules must collide for a reaction to occur. Hence, the rate of a reaction is proportional to the number of collisions per second. This number, in turn, is proportional to the concentrations of the species combining. Normally, chemical equations, like the one given above, are stoichiometric statements. The coefficients in the equation give the number of moles of reactants and products. However, if (and only if) the chemical equation is also valid in terms of what the molecules are doing, the reaction is said to be an elementary reaction. In this case we can write the rate laws for the forward and reverse reactions as Vf = kf[A]"[B]6 and vr = kr[C]c, respectively, where kj and kr are rate constants and the exponents are equal to the coefficients in the balanced chemical equation. The net reaction rate, r, for an elementary reaction represented by Eq. 2.32 is thus... 

The values of the primary quantum yields, found in the photolysis of ketones with y-H atoms, were explained by Brunet and Noyes on the basis of steric effects that diminish the probability of the formation of the cyclic complex. Following the original suggestion of Whiteway and Masson, Martin and Pitts proposed the internal conversion of the cyclic structure to be responsible for the low primary quantum yields observed in the photolysis of ketones capable of forming such a structure. In contrast to other interpretations. Wagner and Hammond explained the low quantum yields by an elementary chemical process, suggesting that the y-H atom transfer is reversible, i.e. that the biradical, after vibrational relaxation, may convert back into the ground state ketone molecule, viz. 

The difference in the elementary scattering process for X-rays and for electrons is also manifested in the differing applicability of the two methods to various problems. X-rays are scattered only by the electrons of the atom, the atomic nucleus, owing to its great mass,... 

Lee, Y.T. Reactive scattering I Nonoptical methods. In Atomic and Molecular Beam Methods. Scoles, G., Ed., Oxford University Press, New York, 1987, Vol. 1, 553-568. Lee, Y.T. Molecular beam studies of elementary chemical processes. In Nobel Lectures in Chemistry 1981-1990. Fraegsmyr, T., Malstrom, B.G., Eds., World Scientific, Singapore, 1992, 320-357. 

See, for example, R. D. Levine and R. B. Bernstein, Molecular Reaction Dynamics. Oxford, New York, 1974. E. E. Nikitin, Theory of Elementary Atomic and Molecular Processes in Gases. Oxford University Press, Oxford, 1974. 

Understanding, at the molecular level, any elementary biological process usually requires the consideration of a large system made of thousands of atoms. This is true for macromolecules such as proteins or nucleic acids, but also for smaller molecules because they can hardly be considered independently of their usually complex surroundings, made of a large number of water, solvent, or host molecules. 

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