Time resolved studies of OH abstraction reactions

A series of studies by Tully and coworkers [5-8] illustrate the technique of laser-flash photolysis and time-resolved spectroscopic detection used for the study of a number of classes of elementary reactions including those of H-atom abstraction by OH or other radicals (Fig. 2.3). The example we shall consider is the reaction of OH with a number of ethane isotopo-mers, e.g.. 

In general the kinetics of the system and the experimental complexity are greatly reduced by having the stable ethane reagent in great excess, making 

Pseudo-first-order rate coefficients are obtained from the gradient of a plot of ln[OH], vs t, e.g., Fig. 2.4. If the pseudo-first-order rate coefficient is determined for a number of different ethane concentrations, then the bimolecular rate coefficient for reaction (10) will be the gradient of a plot of k vs [C2H6]. 

OH radicals can be generated from a number of different precursors. In one of their studies Tully et al. [7] used an indirect method, first producing excited oxygen (0( D)) atoms from the photolysis of nitrous oxide with 193 nm photons from a nanosecond pulse from an excimer laser 

Substrate concentrations are chosen such that OH is generated on a time-scale that is short in comparison to its decay via reaction (10). Alternative methods include direct photolysis of suitable precursors, e.g.. 

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