Three-body association formation

Ion-molecule radiative association reactions have been studied in the laboratory using an assortment of trapping and beam techniques.30,31,90 Many more radiative association rate coefficients have been deduced from studies of three-body association reactions plus estimates of the collisional and radiative stabilization rates.91 Radiative association rates have been studied theoretically via an assortment of statistical methods.31,90,96 Some theoretical approaches use the RRKM method to determine complex lifetimes others are based on microscopic reversibility between formation and destruction of the complex. The latter methods can be subdivided according to how rigorously they conserve angular momentum without such conservation the method reduces to a thermal approximation—with rigorous conservation, the term phase space is utilized. 

Under normal circumstances, this occurs by collisions with a third-body species and the reaction rate therefore depends on total pressure. Such a mechanism is impossible in the super-rarified environment of interstellar space. However, the kinetics of such reactions are of indirect interest to astrochemists on two counts. First, treatments of radiative association [22], which is implicated in the formation of molecular species in interstellar clouds, have much in common with those of three-body association [23]. Second, the rate constants for radical association in the limit of high pressure correspond to those for formation of the energised associated molecule, since all such species are collisionally stabilised in the limit of high pressure. Consequently, the values of kggg and how they vary with temperature provide an important test of theories of reactions occurring over attractive potential energy surfaces [6]. 

Apart from the very dense inner zone, all reactions in disks are two-body processes. Three-body reactions become competitive only at <10AU, where n > 1010 cm-3 (Aikawa el al. 1999). The processes leading to formation of molecular bonds are radiative association, associative detachment, and surface reactions. Reactions of associative detachment are not efficient despite their high reaction rates (ao 10-9 cm-3 s 1), mainly due to low abundances of negative ions (but see also Herbst 1981 Millar et al. 2000 McCarthy et al. 2006). 

At this point it is worth emphasizing that three-body effects can be observed in a gas of light-heavy dimers, where the interdimer repulsion originating from the exchange of the light fermions strongly reduces the decay rate associated with the relaxation of the dimers into deep bound states. The trimer formation in dimer-dimer collisions is... 

Reactions in RC Ionization is initiated by formation of alkah-metal ions on the emitter. A successful method for obtaining lithium ion cationization mass spectra must provide a means for binding hthium ion to the sample molecules. Lithium ions can be injected into an inert gas, such as nitrogen, that contains a trace amormt of sample, and (M + Li)+ complexes are formed by termolecnlar association reactions. The ionic products observed are a reflection of association reaction rates. The formation of an adduct ion is commonly assumed to be a three-body process, in which a neutral molecule collides with an ion-molecule complex and removes an amormt of energy, stabilizing the (M + Li)+ complex. Based on this model, the cationization process would be suitable in a high-pressure environment, since the added gas molecules serve as the third body in the analyte addition reaction with alkali-metal ions. 

As we have discussed earlier, with the two-body potentials of equations (1) and (2), it was necessary to invoke a special set of rules to allow only free atoms and diatomic molecules in the system, and not trimers, quadrumers, etc. These rules were arbitrary, and may not give a proper phenomenological description of the dissociation and the formation of diatomic molecules. It was of interest to introduce three-body interactions, following Murrell, et al. [6], as described in Section 2, to see what difference they would make in the dissociation and association processes. 

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