Stabilization radiative

The stabilization of a quasi-molecule can occur in two ways by radiation of light (radiative stabilization) and as a result of collision (collisional stabilization). 

Radiative stabilization of a most simple kind was first observed [235] on heating of chlorine, bromine and iodine vapour to a temperature above lOOO C. The luminescence spectra of all halogen vapours were found to represent an inversion of their absorption spectra being consistent with process X + X - Xg hv which is an inversion of photodissociation Xg -f- hv X + X. 

Radiative recombination is a very slow process. It has been deduced from experimental data that the probability P,. of a bromine-atom radiative recombination per collision is of the order of 10 [235]. However, despite this value, radiative recombination is manifested in many low-density systems and sometimes plays the crucial role in chemical transformations occurring in such systems. The continuous luminescence spectra observed for various flames are undoubtedly due mostly to recombination processes, i.e. to radiative stabilization of quasi-molecules formed by collisions of atoms and radicals with each other or with molecules present in the combustion zone (see e.g. [153, 359]). Radiative recombination determines the rate of the formation of molecules in interstellar clouds [102, 514] and the spectra of gas discharge in molecular gases [89, 472]. 

Quantitative experimental information concerning the rate of bimolecular reactive recombination is rather sparse. This is mainly attributed to the difficulties in the identification of the observed continuous spectra and to effective 

In the case of atomic recombination, the lack of experimental data is, in a way, compensated by recent quantitative theoretical calculations based on detailed information on the electronic states of diatomic species (see reviews [442, 496]). 

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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. 

In specific applications, it is critically important to know which isomer is produced in a particular situation in order to ascertain its further reactivity. Indeed, further reactivity, in the form of rate coefficients and product ion distributions, both identifies which reactions generate the same isomeric forms and gives information to enable the isomeric forms to be identified (often by determining the energetics and comparing them with theoretical calculations). One such application is to molecular synthesis in interstellar gas clouds. In the synthesis of the >115 molecules (mainly neutral -85%) detected in these clouds,14 a major production route is via the radiatively stabilized analog of the collisional association discussed above,15 viz. ... 

However, again the isomeric form of the product ion is not known.24 For this, Jarrold et al.76 concluded that, under low-pressure conditions, unimolecular decomposition of the C2HjO+ will occur giving HCO++CH4 (as expected from inspection of Figure 5) and that radiatively stabilized association is unlikely. 

Dunbar has added a deep theoretical imderstanding to the radiative stabilization process and has shown, using his standard hydrocarbon model, that the radiative cooling of chemically activated adduct ions, with energy e" in the /th level of the th normal mode, can be very accurately modeled by use of Equation (12). ... 

Bass et al. (1981) published phase space theory models of the reaction CHj + HCN -> (CH3 HCN) + hv, analyzing, in particular, radiative stabilization of the complex. Important work on radiative stabilization has also been published by Dunbar (1975), Herbst (1976) and Woodin and Beauchamp (1979). 

A versatile route for the gas-phase synthesis of various gold(I) complexes is provided by the reaction of Au+ with hexafluorobenzene. While IE F ) = 9.91 eV is large enough to prevent ET, C6F6 has a sufficient number of rovibronic states to allow for efficient formation of the Au(C6F6)+ complex via radiative stabilization in the low-pressure regime according to Reaction (7.5) (Schroder et al. 1995) ... 

Chemical reactions take place in ionized clusters containing acetylene or other triple-bonded moieties. The reaction heat liberated upon formation of the new bond(s) is dissipated by condensation (i.e. elimination of a neutral residue from the cluster), unless collisional or radiative stabilization is possible. 

It was experimentally found that the overall process of ion-molecule association reactions can more descriptively be schemed [1] as follows collisional stabilization reaction (Eq. 2.1), radiative stabilization reaction (Eq. 2.2), and elimination reaction (Eq. 2.3) from ion-molecnle complexes. 

Pressure regimes exist if the rate for complex redissociation k j exceeds the rate for radiative stabilization kj. which, as will be discussed below, is thought to be probably at least around 1(3) s" In this circumstance every complex formed w l not be stabilized by the radiative mechanism alone. The first of three pressure regimes to be considered is the low pressure or radiative regime, which is defined by the relation kj. kg[Q. Here radiative stabilization is more rapid than collisional stabilization. If one assumes coUiaonal staWlization to occur at tihe collisional limiting value of - l(-9) cm s and k to be 1(3) s, the inequality becomes [C] 1(12) cm. Under these conditions, equation (7) reduces to the much sin ler relation... 

In order to calculate radiative assodation rate co dents to me accuracy obtained in me ternary case or to use ternary data to estimate radiative association rate coeffidents, it is necessary to determine me radiative stabilization rate of the complex k Befoe attempting to calculate the radiative stabilization rate me specific mechanism involved must be known. Until quite tecendy, it was dmught (Herbst 1982c, 1985a) diat me only general... 

It has been understood for some dme that vibrational emission is not the only mechanism by which radiative stabilization of the complex can occur. If the complex is formed in an excited electronic state, then emission to stable vibrational levels of the ground electronic state is a possibility. Since electronic transidons typically occur at frequencies larger dian vibradonal transidons and since the spontaneous transition coefficient contains a factor in which the frequency of emission is cubed, electronic stabilization can be much more rapid than vitotional stabilization if there is a large transition dipole moment between the two electronic states. Calculations on the radiative association between C and H2 have shown that thqrc rpears to be an efficient pathway for production of the complex in its excited state, which can then emit to... 

Although theoretical treatments of ternary association rates are quite successfiil, their success in determining radiative association rates is limited. The key problem is fee calculation of the radiative stabilization rate of fee collision complex. The mechanism by which photons are emitted is uncertain. The few measurements so far undertaken do not give a clear guide. The situation will doubtless be resolved wife both ab initio quantal calculations and a new generation of expoiments including some feat measure the intensity of the radiation emitted in the stabilization process. 

If we label the rate coefficients for formation, dissociation, and radiative stabilization of the complex ki, k-i, and kr respectively, the steady-state approximation yields that the second-order rate coefficient fcra for radiative association is... 

Ne " populated during collisions between Ar and O or Ne °", respectively [79, 80]. We have determined the degree of radiative stabilization of these doubly-excited states and compared it with the experimental results. 

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