Atomic recombination dynamics

The approach is ideally suited to the study of IVR on fast timescales, which is the most important primary process in imimolecular reactions. The application of high-resolution rovibrational overtone spectroscopy to this problem has been extensively demonstrated. Effective Hamiltonian analyses alone are insufficient, as has been demonstrated by explicit quantum dynamical models based on ab initio theory [95]. The fast IVR characteristic of the CH cliromophore in various molecular environments is probably the most comprehensively studied example of the kind [96] (see chapter A3.13). The importance of this question to chemical kinetics can perhaps best be illustrated with the following examples. The atom recombination reaction... 

To examine the soUd as it approaches equUibrium (atom energies of 0.025 eV) requires molecular dynamic simulations. Molecular dynamic (MD) simulations foUow the spatial and temporal evolution of atoms in a cascade as the atoms regain thermal equiUbrium in about 10 ps. By use of MD, one can foUow the physical and chemical effects that induence the final cascade state. Molecular dynamics have been used to study a variety of cascade phenomena. These include defect evolution, recombination dynamics, Hquid-like core effects, and final defect states. MD programs have also been used to model sputtering processes. 

The first reaction filmed by X-rays was the recombination of photodisso-ciated iodine in a CCI4 solution [18, 19, 49]. As this reaction is considered a prototype chemical reaction, a considerable effort was made to study it. Experimental techniques such as linear [50-52] and nonlinear [53-55] spectroscopy were used, as well as theoretical methods such as quantum chemistry [56] and molecular dynamics simulation [57]. A fair understanding of the dissociation and recombination dynamics resulted. However, a fascinating challenge remained to film atomic motions during the reaction. This was done in the following way. 

VI. Pair Recombination—A Stochastic Approach A. Generalized Encounter Theory An Inelastic Collision Integral Atomic and Molecular Recombination Dynamics... 

Consider the case of atomic recombination in which hopping between electronic surfaces is suppressed. Thus the dynamics take place on a single electronic surface, which gives rise to an effective potential characterized by a transition state at Rj. 

This type of model is not especially appropriate in all situations, however—for example, the atom recombination process. The relevant potential energy surface for this process was shown in Fig. 4.1. We noted in Section IV that there are no large chemical barriers for the recombination, and the details of the strongly attractive forces play an essential role in the reaction dynamics of this system. A theoretical treatment of this reaction must therefore include this direct chemical force, since it will play an essential role in governing the dynamics of the approach of the atoms through the solvent. We shall defer a thorough discussion of this case to Section XII, where the atom recombination problem is discussed in more detail, but the kinetic theory is formulated in a way that permits this case also to be studied. 

In the 1970s and 1980s both the clean and H-covered Si surfaces were characterized by diffraction and spectroscopic methods, but only in the last decade have there been reproducible studies of chemical kinetics and dynamics on well-characterized silicon surfaces. Despite the conceptual simplicity of hydrogen as an adsorbate, this system has turned out to be rich and complex, revealing new principles of surface chemistry that are not typical of reactions on metal surfaces. For example, the desorption of hydrogen, in which two adsorbed H atoms recombine to form H2, is approximately first order in H coverage on the Si(lOO) surface. This result is unexpected for an elementary reaction between two atoms, and recombi-native desorption on metals is typically second order. The fact that first-order desorption kinetics has now been observed on a number of covalent surfaces demonstrates its broader significance. 

It turns out that the found mechanism is not as schematically simple as the SDDJ model, and it is more appropriate to be referred to as coupled proton-electron transfer in excited state , rather than hydrogen-atom transfer . This mechanism further requires the study of recombination dynamics between the separated charges in clusters and solvents. 

A. J. Stace and J. N. Murrell, Mol. Phys., 33,1 (1977). Molecular Dynamics and Chemical Reactivity. A Computer Study of Iodine Atom Recombination under High Pressure Conditions. 

The iodine-atom recombination, despite its apparent simplicity at first sight, is not ideally suited to such studies, partly because of its spectroscopic peculiarities. Such complexities may well make accurate theoretical treatments very difficult, and compel resort to molecular-dynamics simulation methods. These have in fact been applied with considerable success [16]. 

Moleculair dynamics calculations have been carried out (Bado et al. 1982) from trajectory simulations of two iodine atoms surrounded by fifty xenon atoms these authors were able to take account of solvent caging, atomic recombination and vibrational relaxation to the solvent. Tramsient electronic absorption spectra of iodine were calculated during the first 800 ps following laser excitation. From these spectra, transient kinetics at any wavelength cam be obtained. 

Advances in pulse radiolysis studies in the gas phase have been summarized in several review papers. In a comprehensive review by Sauer [4], a review presented by Firestone and Dorfman [5] in 1971 was referred to as the first review on gas-phase pulse radiolysis. Experimental techniques and results obtained were summarized by one of the present authors [6], with emphasis on an important contribution of pulse radiolysis to gas-phase reaction dynamics studies. Examples were chosen by Sauer [7] from the literature prior to 1981 to show the types of species that were investigated in the gas phase using pulse radiolysis technique. Armstrong [8] reviewed experimental data obtained from gas-phase pulse radiolysis together with those from ordinary steady-state radiolysis. Advances in gas-phase pulse radiolysis studies since 1981 were also briefly reviewed by Jonah et al. [9], with emphasis on an important contribution of this technique to free radical reaction studies. One of the present authors reviewed comprehensively the gas-phase collision dynamics studies of low-energy electrons, ions, excited atoms and molecules, and free radicals by means of pulse radiolysis method [1-3]. An important contribution of pulse radiolysis to electron attachment, recombination, and Penning collision studies was also reviewed in Refs. 10-15. 

We present a preliminary study on the structural dynamics of photo-excited iodine in methanol. At early time delays after dissociation, 1 - 10 ns, the change in the diffracted intensity AS(q, t) is oscillatory and the high-q part 4 -8 A 1 is assigned to free iodine atoms. At later times, 10-100 ns, expansive motion is seen in the bulk liquid. The expansion is driven by energy released from the recombination of iodine atoms. The AS(q, t) curves between 0.1 and 5 (is coincide with the temperature differential dS/dT for static methanol with a temperature rise of 2.5 K. However, this temperature is five times greater than the temperature deduced from the energy of dissociated atoms at 1 ns. The discrepancy is ascribed to a short-lived state that recombines on the sub-nanosecond time scale. 

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