Detachment, associative

Fehsenfeld F C 1975 Associative Detachment Interactions Between Ions and Molecules ed P Ausloos (New York Plenum)... 

Viggiano A A and Paulson J F 1983 Temperature dependence of associative detachment reactions J. Chem. Phys. 79 2241-5... 

Recent laboratory studies have concentrated on the loss mechanisms for the primary negative ions. The process of associative detachment ... 

Fehsenfeld, F. C. Associative detachment. In Interactions between ions and molecules. Ausloos, P. (ed.), pp. 387-412. New York Plenum Press 1975... 

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). 

In denser clouds, however, the abundance of H2 increases rapidly. With gas densities > 102 cm-3 and column densities corresponding to an extinction in the visible of Av > 0.5m hydrogen is expected to be mainly in the molecular form (Werner et al., 1970). Similar results are obtained by de Jong (1972) who also includes low-energy cosmic radiation in his calculations which produces electron densities in dark clouds sufficient to yield H2 formation via associative detachment H- + H -> H2 + e proceeding with a rate constant k v 1.3 x 10 9 cm3 sec-1 at 300 °K. This reaction, however, is only of importance if formation of H2 molecules on grains is for one reason or another inefficient. 

Associative reaction (19.65) has been studied experimentally for v = 0, and its total (unresolved in V3) cross-section is known to very low ( 0.05eV) collision energies [35]. Associative detachment cross-section (as well as that for associative ionization of neutral particles) shows an E-1 energy dependence at collision energies below 1 — 2eV. Collision processes (19.66a) and (19.66b) of 11 with H3 have not been studied as yet. It is anticipated that... 

For the case of high temperatures found in the vicinity of hot stars, or in cloud regions under the influence of a shock wave, or in ionized nebulae, several authors have suggested (c.f. Jura, 1975, Dalgamo and McCray 1973) a gas phase formation scheme which could be a significant source for molecular hydrogen, Hj. The associative detachment reaction... 

The low effective value for the direct dissociative attachment process was later shown to be due, in part, to the regeneration of electrons via the associative detachment reaction,... 

Several exothermic associative detachment reactions do not occur at measurable rates (k < 10 12 cc./sec.), an example being... 

Several processes are unique to ions. A common reaction type in which no chemical rearrangement occurs but rather an electron is transferred to a positive ion or from a negative ion is termed charge transfer or electron transfer. Proton transfer is also common in both positive and negative ion reactions. Many proton- and electron-transfer reactions occur at or near the collision rate [72]. A reaction pertaining only to negative ions is associative detachment [73, 74],... 

Associative detachment reactions are important in controlling the electron concentration in the earth s mesosphere [75]. Reactions in which more than one neutral product are formed also occur and are sometimes referred to as reactive detachment [76]. 

Destruction of Negative Ions Associative Detachment, Electron Impact Detachment, and Detachment in Collisions with Excited Particles... 

Different mechanisms of destmction of negative ions releasing an electron are discnssed in special books by Massey (1976), McDaniel (1964), and Smirnov (1982). We are going to consider three detachment mechanisms most important in plasma-chemical systems. The first one, which is especially important in non-thermal discharges, is associative detachment ... 

This is the reverse process with respect to the dissociative attachment (2-66) and therefore it can also be illnstrated by Fig. 2-7. The associative detachment is a non-adiabatic process, which occnrs via intersection of electroiuc terms of a complex negative ion A -B and corresponding molecnle AB. Rate coefficients of the non-adiabatic reactions are qnite high, typically kd = 10 °-10 cm /s. The kinetic data and enthalpy of some associative detachment processes are presented in Table 2-7. 

The energy efficiency of the CO2 dissociation through dissociative attachment is limited by the loss of an electron in the process (the energy price of an electron is quite high, usually 30-100 eV). Getting the electron back in the process of associative detachment. 

Restoration of electrons and chain propagation is provided by ion-molecular reactions and associative detachment, producing additional sulfuric acid ... 

Below cs 300 °C the equilibrium is far on the right-hand-side but the decomposition of carbon monoxide with formation of CO2 and deposition of solid carbon proceeds only at an extremely slow rate because of the high dissociation energy of CO A weak discharge operating at a current density of a few mA/cm efficiently enhances the reaction rate the probable reaction mechanism being the dissociative attachment on CO followed by associative detachment of 0 and CO... 

Reactions which are the reverse of those mentioned here as preparative techniques are important negative-ion loss processes under some conditions. Associative detachment, collisional detachment, ion-ion recombination, and photodetachment are the reverse processes to those discussed in Sections 2.1-2.4. Several reviews of these subjects are available. ... 

The remainder of this chapter is devoted to a discussion of some of the results obtained from studies of negative-ion-neutral reactions. No attempt is made to include extensive discussion either of associative detachment processes (see Chapter 8) or of those many negative-ion reactions which have been studied only at relatively high energies, i.e., above a few hundred eV. Two-body rate coefficients here are invariably given in units of cm sec and cross sections in units of cm. Energies refer to kinetic energies of the reactant ion in the laboratory system, unless otherwise stated. 

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