Rate coefficient with excited species

Basco studied the reaction in a similar flash photolysis apparatus, and found the overall rate coefficient for CN disappearance to be 4.6 x 10 l.mole . sec , in excellent agreement with Paul and Dalby . He found that in the presence of excess O2, ozone was formed. He also spectroscopically detected NCO, a previously unidentified species which he assumed to be N2C2O or NCO2CN, and vi-brationally excited NO. To explain these results, he proposed the mechanism... 

How may these measured functions be related to the rate coefficients of reactions of the excited species It is useful to consider how the excitation arises and the various ways in which the excited species may dissipate its excess energy. Suppose A is excited to A which reacts with B to give products. There are two distinct mechanisms which may be considered for such a fluorescence quenching process. Before the transfer of energy from A to B is possible the species must form an encoimter complex in which the solvent cages of A and B have been sufficiently modified to allow significant chemical interaction between the two reactants. By the dynamic, or diffusional, pathway A is formed in comparative isolation from B, and the encounter complex is produced as a second step... 

More importantly, a molecular species A can exist in many quantum states in fact the very nature of the required activation energy implies that several excited nuclear states participate. It is intuitively expected that individual vibrational states of the reactant will correspond to different reaction rates, so the appearance of a single macroscopic rate coefficient is not obvious. If such a constant rate is observed experimentally, it may mean that the process is dominated by just one nuclear state, or, more likely, that the observed macroscopic rate coefficient is an average over many microscopic rates. In the latter case k = Piki, where ki are rates associated with individual states and Pi are the corresponding probabilities to be in these states. The rate coefficient k is therefore time-independent provided that the probabilities Pi remain constant during the process. The situation in which the relative populations of individual molecular states remains constant even if the overall population declines is sometimes referred to as a quasi steady state. This can happen when the relaxation process that maintains thermal equilibrium between molecular states is fast relative to the chemical process studied. In this case Pi remain thermal (Boltzmann) probabilities at all times. We have made such assumptions in earlier chapters see Sections 10.3.2 and 12.4.2. We will see below that this is one of the conditions for the validity of the so-called transition state theory of chemical rates. We also show below that this can sometime happen also under conditions where the time-independent probabilities Pi do not correspond to a Boltzmann distribution. 

A number of molecular species, such as NF3, HCl, HBr, HF, and N2O, exhibit strong dependence of attachment rate on temperature. This comes about because the dissociative attachment cross section increases with increasing vibrational quantum number (see Christophorou et al, 1994). This can be seen in the temperature dependence of the N2O dissociative attachment cross section shown in Fig. la, and its effect on the attachment rate coefficient can be seen in Fig. lb. The attachment cross section (Christophorou et al, 1971), where the products are NO + 0, is very temperature-dependent (Chantry, 1969), as shown in Fig. lb, which means that it is very sensitive to the degree of vibrational excitation. In a plasma, one does not need an elevated gas temperature to populate the molecular... 

Comparing direct ionization (2-18) with stepwise ioiuzation (2-25), one can see that the second one can be mnch faster becanse of the high statistical weight of excited species involved in the stepwise ioiuzation. The ratio of rate coefficients for these two competing mechanisms of ionization can be easily derived from (2-18) and (2-25) ... 

If the rate of interphase exchange is comparable with the rates of the reactions and decay of excited molecules, one can use equation similar to Eq. (54), taking into account the value of the coefficient d, which determines the average excess of the concentration C in a micelle during the lifetime of the excited species ... 

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