Alkali reaction with halogen molecules

The reactions of alkali dimers with halogen molecules also exhibit a multiplicity of reaction pathways with production of electronically excited atoms, electronically excited molecules, and ions all apparently possible [364, 367-370], In these reactions, however, the dominant pathway, with a reactive cross section >150 A2, leads to products in their electronic ground... 

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. 

The reactions44-45 of alkali dimers with halogen molecules offer an opportunity to study the dynamics of four-centre reactions. Three-centre reactions of an atom plus diatomic molecule have been extensively studied and also four-centre reactions46 of an atom plus triatomic molecule. However, there are few examples known47 of reactive four-centre diatomic plus diatomic systems. 

Typical reactions with an early barrier studied by molecular beams include those of the alkali metals with halogens and other simple molecules. 

Despite an extensive study of the reactions of alkali dimers (see the reviews of Grice [208] and Herm [216]), there have been few direct measurements of energy disposal in these systems. The chemiluminescent reactions of oxygen and halogen atoms with alkali dimers have already been described [Sects. 3.1.7(c) and 3.1.8(b)], as have the alkali atom-alkali dimer exchange reactions [Sect. 3.1.2(b)], In this section we describe the chemiluminescent reactions of alkali diatomic molecules with halogen molecules. 

These reactions were pioneered by Grice and co-workers who first addressed the reactivity of alkali metal dimers with halogen molecules [108-111]. These reactions are four-center harpoon reactions. They are interesting because of the possibility of two sequential electron jumps [112. ... 

The study of alkali atom reactions with halogen-containing molecules comprises much of the history of reactive scattering in molecular beams. The broad features of the reaction dynamics and their relation to the electronic structure of the potential energy surface are well understood.2 The reaction is initiated by an electron jump transition in which the valence electron of the alkali atom M is transferred to the halogen-containing molecule RX. Subsequent interaction of the alkali ion and the molecule anion, in the exit valley of the potential surface, leads to an alkali halide product molecule MX. 

The reaction of halogen molecules (X = Cl, Br, 1) with cold aqueous alkali to yield hypo-halite (OX ) and halide (X ) may be viewed as a classic inorganic 8 2 displacement, rather similar to the reaction with pyridine mentioned above ... 

Very detailed reactive scattering studies of electronically excited alkali atoms with HQ, O2, NO2, etc. have been reported [41], and the reactions of polarized laser-excited rare gases with halogenated molecules to yield aligned excimer products studied [42]. 

The reactions in Table 1 are transfer of H, O, N, S, halogen and alkali metal atoms. They are also reactions of atoms (H, O, N, S, halogen and alkali metal atoms) with small molecules. They are exothermic and have high specific rates (low activation energies and normal steric factors). These features are desirable for the study of chemi-excitation. High heat release permits substantial excitation. Small, low moment of inertia product molecules have spectra that may be resolved with moderate power spectrographic and spectrometric instruments. High speed provides the necessary number of reaction acts per unit time. 

This effect is best viewed in single harpoon reactions such as those of alkali metal atoms with halogen-containing molecules discussed in Section 2.3.1. A series of studies conducted in a crossed-beam experiment by Lee s group at Berkeley have demonstrated how the electronic excitation of sodium affects the dynamics of these reactions. 

The electron jump mechanism, (see 1), has long been invoked71,2 in a highly simplified manner to explain the stripping dynamics of alkali atom-halogen molecule reactions M + X2. Electron transfer occurs in the entrance valley of the covalent M + X2 potential surface near the intersection with the ionic M+ + X2 potential surface. The M - X2 internuclear distance R at the intersection Rc is roughly estimated... 

Soon after the first experiments on halogen molecules, it became clear that alkali collisions with many other, electronegative and even electropositive, molecules lead to ionization. Many of these results have been published now, especially on relatively small inorganic molecules, whereas experiments on large organic molecules have been announced. Moreover the history of the chemical reaction beam research seems to repeat itself after an alkali-age of about five years the first non-alkali experiments have been reported, which will be discussed in the next section. 

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