Recombination reactions electronic excitation

From inspecting the atomic database of the EIRENE code [31], which is used in many applications to a large number of different tokamaks, including for the ITER design, in particular its collisional-radiative models for molecules, it was clear that matters can be more complicated. The relaxation time for establishing a vibrational distribution of the ground state molecule is comparable to the transport time of the molecule, hence the applicability of local collisional-radiative approximations is questionable. Furthermore, one of the two atoms created in dissociative recombination is electronically excited, and, hence, can be ionized very effectively even at low divertor plasma temperatures (instead of radiative decay). In this case, the whole chain of reactions would be just an enhanced ( molecular activated ) dissociation (MAD, i.e., dissociative excitation of those H]]", which have been produced... 

An interesting type of recombination reaction in excited electronic states is the excimer formation in the mercury-ammonia system. By measuring the relaxation times for the decay of the excimer emission, Call ear and co-workers were able to obtain the rate constants for the reaction ... 

All of the atomic species which may be produced by photon decomposition are present in plasma as well as the ionized states. The number of possible reactions is therefore also increased. As an example, die plasma decomposition of silane, SiH4, leads to the formation of the species, SiH3, SiHa, H, SiH, SiH3+ and H2+. Recombination reactions may occur between the ionized states and electrons to produce dissociated molecules either direcdy, or tlrrough the intermediate formation of excited state molecules. 

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>. 

The simplest systems where electron-transfer chemiluminescence occurs on interaction of radical ions are radical-anion and radical-cation recombination reactions in which the radical ions are produced from the same aromatic hydrocarbon (see D, p. 128) by electrolysis this type of chemiluminescence is also called electro-chemiluminescence. The systems consisting of e.g. a radical anion of an aromatic hydrocarbon and some other electron acceptor such as Wurster s red are more complicated. Recent investigations have concentrated mainly on the energetic requirements for light production and on the primary excited species. 

Since the values of i/ depend on several factors noted above, in the absence of additional data such as the temperature dependence of the electron transfer rate constants for i-2 it is difficult to analyze the apparent difference between i/ for the charge separation reaction and that of the radical ion pair recombination reaction. However, the difference between these two values of u is not unreasonable given that the charge separation involves oxidation of an excited state of the donor, while radical ion pair recombination involves two ground state radicals. Small changes in the nuclear coordinates of the donor and acceptor for these two reactions should be sufficient to produce the observed difference in i/. The electronic coupling factor between ZnTPP and AQ should be different than that between ZnTPP " and AQ". 

Many spectroscopic methods have been employed for the investigation of such systems For example, wide-band, time-resolved, pulsed photoacoustic spectroscopy was employed to study the electron transfer reaction between a triplet magnesium porphyrin and various quinones in polar and nonpolar solvents. Likewise, ultrafast time-resolved anisotropy experiments with [5-(l,4-benzoquinonyl)-10,15,20-triphenylpor-phyrinato]magnesium 16 showed that the photoinduced electron transfer process involving the locally-excited MgP Q state is solvent-independent, while the thermal charge recombination reaction is solvent-dependent . Recently, several examples of quinone-phtha-locyanine systems have also been reported . 

If the recombination is delayed, e.g., by migration of excited electrons, luminescence takes place by a second-order bimolecular reaction. The probability of a luminescent recombination of the excited electron with the holes is then proportional to the product of the concentration of electrons and the concentration of holes. The lower the initial intensity is, and the further the decay has progressed, the slower the decay to the half value is. This hyperbolic decay law is only of limited validity. If the excited electron is momentarily trapped before recombination, very complex interactions can arise. 

Another important process is ion recombination, when a solvated electron meets a positive ion. One of the neutral products can be formed in an electronically excited state and its chemical reactions are then similar to photochemistry... 

In an electron transfer reaction two neutral molecules, for example, will form an ion pair if one of the neutrals is electronically excited it will be deactivated, and the overall process may appear as a quenching if the ion pair recombines to return to the neutral ground state partners ... 

Chemiluminescence can occur when a thermal (dark) reaction is so exothermic that its energy exceeds that of the electronically excited state of one of the product molecules. The major pathway for these reactions is the decomposition of cyclic peroxides, and this is at the basis of most bioluminescence processes. There are some other physico-chemical processes which can lead to the formation of excited states and thereby to the emission of light these are based on the bimolecular recombination of high-energy species such as free radicals and radical ions. 

Another theoretical method widely applied to describing the kinetics of electron tunneling reactions is based on the employment of the so-called conditional concentrations. This method was first suggested by Galanin [5] in the theory of electron excitation energy transfer and by Antonov-Roma-novskii in the theory of particle recombination [17]. 

Due to the rapid decrease in the process probability with increase of the distance between the reagents, it should be expected that reaction (13) will result in electron transfer primarily to the particle A which is nearest to the excited donor particle D. In this case, the condition n < N is satisfied for reaction (13), where n is the concentration of the particles D and N is that of the particles A, and with the random initial distribution of the particles, A, the distribution function over the distances in the pairs D A formed, will have the same form [see Chap. 4, eqn. (13)] as with the non-paired random distribution under the conditions when n IV. In such a situation the kinetics of backward recombination of the particles in the pairs D A [reaction (12)] will be described by eqn. (24) of Chap.4 which coincides with eqn. (35) of Chap. 4 for electron tunneling reactions under a non-paired random distribution of the acceptor particles. Therefore, in the case of the pairwise recombination via electron tunneling considered here, the same methods of determining the parameters ve and ae can be applied as those described in the previous section for the case of the non-pair distribution. However, examples of the reliable determination of the parameters ve and ae for the case of the pairwise recombination using this method are still unknown to us. 

Just as for the reaction of Nh with CC14 [29], no ion formation could be detected as a result of luminescence quenching. This has been explained by subsequent fast recombination reactions of the ions formed. According to the estimation of ref. 34 based on the values for the added quencher concentrations, the distances of electron transfer from the triplet excited state of a TMPD molecule to PA and from HMTI to PMA are equal to about 25 A. 

The physicochemical stage includes the chemical processes in electron excitation states, as well as the chemical transformations of the active intermediates under nonequilibrium conditions. These are the predissociation and the ion-molecular reactions that take about 1013 s the recombination of positive ions with thermalized electrons (1CT12-10 10s) and the electron-solvation reactions (10 12-10-1° s). Thus, the physicochemical stage lasts from 1CT13 to 10-I0s. 

The translational temperature Tt plays an important kinetic role. At high temperatures chemical reactions are fast, and — in view of the decreasing rate of surface recombination of atoms — the energy exchange of the system with the environment becomes slower. Consequently, the theoretical model can be applied to such systems (for comparison see Table 1 in43)). The actual equilibrium concentration of the volatile reaction products — CN in the present case - may be reduced by dissociative de-excitation of electronically excited species (cf. also the system C/H2). 

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