Ion recombination energy

In Sec. 8 we took the point of view that, when a molecule has been dissociated into ions, energy equivalent to the work done may be regarded as stored in the form of potential energy, which will be liberated when the ions recombine. The same point of view can be adopted with regard to the quantities Yvae and Y. The electrostatic part of this energy may be regarded as associated with the ionic field. 

In Table II the state of the ion and the recombination energies in electron volts (computed from (65)) are given. Some very uncertain information is included in the right hand column as to the relative abundances of the metastable states of the ions when produced by electron impact with 100-e.v. electrons from the indicated compounds. 

The eventual fate of any ion is its neutralization, either by a free electron or by a negative ion formed by electron attachment. In ethylene radiolysis at high dose rates, electron capture processes should be insignificant (29), and the recombination energy of the positive ion will become available on neutralization, a portion of which may be in the form of excitation (59). 

Bassi et al. [70] have described IMR-MS for online gas analysis with a sensitivity of 100ppb-l ppm. A mass-selected ion source allows the use of three different primary ion beams (Xe+, Kr+ and CF3I+), covering the recombination energy range from 10.23 to 14.67 eV. For fast measurements, the change from one primary ion to another can be achieved by a Wien filter. IMR-MS allows quantitative analysis. 

Excited states of hydrocarbon molecules often undergo nondissociative transformation, although dissociative transformation is not unknown. In the liquid phase, these excited states are either formed directly or, more often, indirectly by electron-ion or ion-ion recombination. In the latter case, the ultimate fate (e.g., light emission) will be delayed, which offers an experimental window for discrimination. A similar situation exists in liquid argon (and probably other liquefied rare gases), where it has been estimated that -20% of the excitons obtained under high-energy irradiation are formed directly and the rest by recombination (Kubota et al., 1976). 

The kinetics of electron-ion recombination is well described by the diffusion model both for photoionization and for ionization induced by high-energy irradiation. 

These energy-transfer processes are especially interesting in those chemiluminescence reactions where the primary electronically excited product is formed in its triplet state (autoxidation reactions, radical-ion recombination reactions see Sections III and VIII), although some reactions have been reported to involve direct emission from the excited triplet state 14>. 

Chemically inert triplet quenchers e.g. trans-stilbene, anthracene, or pyrene, suppress the characteristic chemiluminescence of radical-ion recombination. When these quenchers are capable of fluorescence, as are anthracene and pyrene, the energy of the radical-ion recombination reaction is used for the excitation of the quencher fluorescence 15°). Trans-stilbene is a chemically inert 162> triplet quencher which is especially efficient where the energy of the first excited triplet state of a primary product is about 0.2 eV above that of trans-stilbene 163>. This condition is realized, for example, in the energy-deficient chemiluminescent system 10-methyl-phenothiazian radical cation and fluoranthene radical anion 164>. 

Alternatively the necessary energy can be supplied by the impact of electrons accelerated through the appropriate potential difference (2) or more subtly by collision with another positive (often atomic) ion with a higher recombination energy (the reverse of its ionization) (3). 

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