Reactions of alkali atoms

The reviews of Grice [208], Herm [216] and Kwei [296] deal with many aspects of the dynamics of alkali atom reactions including energy disposal. Table 2 gives a summary of energy disposal measurements for alkali atom reactions. 

The dynamics of the reactions of alkali atoms with hydrogen halides are constrained by angular momentum conservation to convert almost all the initial orbital angular momentum into rotational angular momentum of the alkali halide product, as mentioned in Sect. 2.2. This is confirmed by electric deflection analyses of the alkali halide products from the reactions K, Rb and Cs + HBr [280—282]. Time-of-flight measurements of the product translational energy distributions for the reactions [278] 

For the reactions K + HBr and K + DBr, the KBr recoil energy distribution has been determined in a crossed-molecular beam experiment using a mechanical velocity selector. No difference was found in the form of the translational energy distributions for the two reactions for which a value of FT 0.30 may be derived. Although all the angular momentum appears in the product rotation, the moments of inertia for the alkali halides are large, which implies that the mean product rotational energy is quite small ( FR 0.21, 0.21 and 0.09 for K, Rb, Cs + HBr, respectively [3] these values are derived from the rotational temperatures obtained by electric deflection analysis). 

Experiments have also been performed to study the effect of reagent translation excitation on the reactivity of K + HC1 [284] and the effect of reagent vibrational and rotational energy for Na + HF, HC1 [65, 285] and K + HC1 [286, 287]. These experiments have not probed the effect on the energy disposal of the increase in reagent energy. The polarisation of the MX product angular momentum has been studied [288] for K, Cs + HBr, HI. 

The only alkali atom—dimer exchange reaction for which product energy distributions have been measured is 

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If the molecules could be detected with 100% efficiency, the fluxes quoted above would lead to impressive detected signal levels. The first generation of reactive scattering experiments concentrated on reactions of alkali atoms, since surface ionization on a hot-wire detector is extremely efficient. Such detectors have been superseded by the universal mass spectrometer detector. For electron-bombardment ionization, the rate of fonnation of the molecular ions can be written as... 

W.B. Miller, S.A. Safron, D.R. Herschbach, Exchange reactions of alkali atoms with alkali halides—A collision complex mechanism, Discuss. Faraday Soc. 44 (1967) 108. 

A widely-used model in this class is the direct-interaction with product repulsion (DIPR) model [173—175], which assumes that a generalised force produces a known total impulse between B and C. The final translational energy of the products is determined by the initial orientation of BC, the repulsive energy released into BC and the form of the repulsive force as the products separate. This latter can be obtained from experiment or may be assumed to take some simple form such as an exponential decay with distance. Another method is to calculate this distribution from the quasi-diatomic reflection approximation often used for photodissociation [176]. This is called the DIPR—DIP model ( distributed as in photodissociation ) and has given good agreement for the product translational and rotational energy distributions from the reactions of alkali atoms with methyl iodide. 

The reactions of alkali atoms with methyl iodide exhibit rebound dynamics in which reaction takes place only at small impact parameters... 

The dynamics of the alkaline earth metal reactions with alkali halides appear to closely resemble the exchange reactions of alkali atoms with alkali halides [208, 216, 296] for which no direct energy disposal measurements have been reported. They proceed through a long-lived collision complex which is identified with a well in the reaction potential-energy surface. 

The polarisation of the CaCl (B) chemiluminescence from the reaction Ca (1D) + HC1 has been measured [379] to determine the rotational alignment of the CaCl product and indicates a highly polarised distribution of product angular momenta. This is similar to the reactions of alkali atoms with hydrogen halides. 

Studies of reactive scattering have been almost entirely restricted until recently to the reactions of alkali atoms with halogen-containing compounds. 

P. Davidovits, Cross-sections for the Reaction of Alkali Atoms with Halogen Molecules, in Alkali Halide Vapors, Structure, Spectra and Reaction Dynamics, edited by P. Davidovits and D.L. McFadden, Chapter 9, p. 331, Academic Press, New York, 1979. 

Calculations of this type have been carried out for the reactions of alkali atoms with methyl iodide 191>, in discussing isotope effects on the abstraction and substitution processes for reactions T + CH4 and T + CD4192), in the classical H+H2 reaction193) for which typical... 

Electron-jump in reactions of alkali atoms is another example of non-adiabatic transitions. Several aspects of this mechanism have been explored in connection with experimental measurements (Herschbach, 1966 Kinsey, 1971). The role of vibrational motion in the electron-jump model has been investigated (Kendall and Grice, 1972) for alkali-dihalide reactions. It was assumed that the transition is sudden, and that reaction probabilities are proportional to the overlap (Franck-Condon) integral between vibrational wavefunctions of the dihalide X2 and vibrational or continuum wave-functions of the negative ion X2. Related calculations have been carried out by Grice and Herschbach (1973). Further developments on the electron-jump mechanism may be expected from analytical extensions of the Landau-Zener-Stueckelberg formula (Nikitin and Ovchinnikova, 1972 Delos and Thorson, 1972), and from computational studies with realistic atom-atom potentials (Evans and Lane, 1973 Redmon and Micha, 1974). 

A few results on polyatomic halogen compounds have been reported. The chemical reaction of alkali atoms with methyl iodide... 

Instable uncharged free radicals and inorganic radical ions Mostly irradiation of solid or liquid substances, also reactions of alkali atoms with organic molecules or reactions of organic compounds with other free radicals. 

The reaction dynamics of systems composed of alkali atoms are remarkably complex, and thus interesting. This is illustrated by trajectory studies of the reactions of alkali atoms with alkali molecules. For example, a trajectory study of the details of the dynamics of collisions of Li with Liz has been done for an ab initio-h ssd PES. The statistical and non-statistical behaviors in the reaction were thoroughly explored in this study. There are two different reaction mechanisms, one direct (i.e., non-statistical) and one in which a collision complex is formed (thus, statistical). The alkalis react on surfaces without potential energy barriers, and with potential wells that accommodate the formation of complexes. Thus, the dynamics of the reaction are strongly dependent on the initial conditions. The statistical mechanism, that is, complex formation, tends to dominate at low translational and vibrational energies whereas the reaction becomes direct as the energy of the system increases. 

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