Radiative association kinetics constant

Again the radiative association kinetics described above allow a direct comparison for some realistic values of k and k. For most chemically activated systems at the threshold for unimolecular dissociation, the observed radiative rate constants are of the order of 10-100 s and hence are much below the values expected for k of about 10 s . Therefore, the first limit is most likely to be valid, with the interesting conclusion that the observed unimolecular dissociation rate constant will depend only on the photon density and the absorption cross section (rate constant) at a given wavelength. 

In this reaction scheme, CH4 is produced by two steps of radiative association with slow rate constants. Because the destruction of C by electron capture (radiative) is four orders of magnitude slower than the destruction of a molecular ion by dissociative recombination, there is not a rapid loss of C , allowing production of a saturated hydrocarbon. We recall that chemical equilibrium arguments predict preponderant conversion of carbon monoxide to methane and water. There is little evidence for this, as stated earlier. The gas-phase production of CH4 from CO and H2 then proceeds by a very high-energy kinetic path, namely He + CO = C +... 

Radiative Association Reactions The study of radiative association reactions, (Eq. 2.2), has been of considerable interest [6-8] in chemical kinetics, planetary and interstellar chemistry, flames, and a variety of other areas. The kinetic study makes it possible to model the formation of complex molecular species in the interstellar science. At the very low molecular number densities in interstellar environments, the probability of formation of the products of association reactions by collisional stabilization is very low. Therefore, the radiative association process becomes an extremely important one for the production of the complex molecular species observed by astronomical physicist. The methodology is either flowing afterglow (FA) or Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. For the study of the apparent bimolecular rate constant for formation of association products as a function of pressme of a third body (N), the pressure should be set up to be sufficiently high in order to release the energy in the associated complex. Under the high pressure conditions collisional stabilization has competed with and usually dominated over radiative associatioiL As a result, the radiative association rate was then extrapolated from the intercept of a plot of apparent rate constant versus pressure of a third body, N. 

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]. 

The kinetics of ion/molecule adduct formation, under the conditions of low-pressure [ion cyclotron resonance (ICR)], is controlled by radiative relaxation the energy released by the complexation can only relax by emission of infrared photons. The link between the strength of the incipient bond and the association rate constants relies on models based on infrared emission for the relaxation of the energy of association [125, 126]. 

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