Radiative Association Competition

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. 

Based on the study of simple radiative association processes that are statistical in nature, one can conclude that even with small binding energies, as the size of the reactants becomes sufficiently large, radiative association becomes 100% efficient. Using the phase-space approach,96 Herbst and Dunbar99 have studied the rate of radiative association reactions between hydrogen-rich hydrocarbons of the type, 

Herbst and Dunbar have investigated the effects of exit channel barriers on association reactions of type 43 and have shown that, depending on the size of the barrier, the efficiency of radiative association reactions as a function of N can be strongly curtailed. For example, at 10 K and a nonpolar neutral reactant, they found for a system with a well depth of 2 eV and an exothermic channel barrier of 1.0 eV, N = 130 atoms for 100% sticking efficiency, approximately 10 times the corresponding value of N in the absence of a competitive exothermic channel. 

There is now known to be a second mechanism allowing competition between association and reaction, which can be termed the parallel mechanism. In this 

Since two mechanisms are possible for the competition between association and reaction, detailed ab initio calculations of the potential surface are even more necessary in theoretical determinations of the rates of association channels. More experimental work is also needed it is possible that as a larger number of competitive systems is studied, our understanding of the competition will increase. Critical systems for interstellar modeling include the association/reactive channels for C+ and bare carbon clusters, as well as for hydrocarbon ions and H2. 

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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). 

Gas-phase metal-ligand bond energies can be measured by a variety of experimental techniques. Measurements of absolute values can be made by temperature-dependent equilibrium methods, " " blackbody infrared radiative dissociation (BIRD), " radiative association, " and the TCID method discussed in detail here. Measurements of relative thermochemistry can be accomplished using equilibrium methods, the kinetic method, " and competitive CID (see Section 2.12.5.7). This review cannot include the details of all such measurements. 

The association of C+ and H2 has a calculated rate coefficient, derived from a statistical theory, of 4 x f0 (T/300) °- cm s in the range fO-300K. ° Although the rate coefficient is small, the fact that H2 is a reactant makes the process competitive. A variety of other radiative association processes are important in interstellar chemistry, perhaps the simplest being the corresponding reaction involving neutral atomic carbon ... 

In addition, a number of organic species are produced from precursor ions that are the products of radiative association reactions. Although radiative association is most important when H2 is a reactant because of the overwhelming relative abundance of H2, such processes between two heavy species are also valuable on occasion. There are some neutral-neutral systems that have been studied, but the majority of these systems are thought to be ion-molecule ones. For the most part, the systems do not possess competitive exothermic channels, although there are exceptions,... 

This reaction has been studied several times in rf ion traps with increasing accuracy. The results are summarized in Fig. 3.22 (see also Refs. 15 and 55). For P-H2 the rate coefHcient for radiative association is 1.7 x 10 cm s at 10 K, while the value for n-H2 is 2.5 times smaller. As discussed in detail in Ref. 22, much more has been learned about such processes, e.g. the competition between complex life time and radiative decay, by comparing ternary and radiative association and by isotopic substitution. 

Understanding the field enhancement of radiative rates is insufficient to predict how molecular photophysical properties such as enhancement of fluorescence quantum yield will be affected by interactions of the molecule with plasmons. A more detailed model of the photophysics that accounts for non-radiative rates is necessary to deduce effects on photoluminescence (PL) yields. Such a model must include decay pathways present in the absence of metal nanoparticles as well as additional pathtvays such as charge transfer quenching that are associated with the introduction of the metal particles. Schematically, we depict the simplest conceivable model in Figure 19. IB. Note that both the contributions of radiative rate enhancement and the excited state quenching by proximity to the metal surface will depend on distance of the chromophore from the metal assembly. In most circumstances, one expects the optimal distance of the chromophores from the surface to be dictated by the competition between quenching when it is too close and reduction of enhancement when it is too far. The amount of PL will be increased both due to absorption enhancement and emissive rate enhancement. Hence, it is possible to increase PL substantially even for molecules with 100 % fluorescence yield in the absence of metal nanoparticles. 

On a time scale of 10 -10 fs two more deactivation processes start to be competitive fluorescence emission (FE) and internal conversion (1C) [62]. The fluorescence emission is simply the radiative process associated with the emission of one photon and... 

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