The starting point for generating detailed reaction mechanisms is knowing the reaction rate coefficients of the basic reactions on which simulation of a chemical kinetic system is based. The aim of experiments into elementary kinetic reactions and reaction systems, e.g. behind shock waves, and of quantum chemical computations, is to describe how the rate coefficient k is dependent on temperature k = k(T) or on temperature and pressure k = k(T,p) within the relevant parameter range.
In the experiments behind shock waves, the attempt is made to start a reaction system in a controlled manner and to achieve a maximum reduction in the effect of subsequent reactions by means of a high level of dilution with inert gas. A shock wave’s characteristic of compressing a gas within a microsecond and raising it to a high temperature state is exploited here in order to initialise the reaction system. By using radical precursor molecules that have one or several radical locations after a thermally induced bond fracture, it is possible for reactions to be started immediately behind the shock wave. Reaction progress is tracked, e.g. by analysing the reactants or transient (generally radical) intermediate products. Spectroscopic methods are preferably used for this purpose, such as atomic/molecular resonance absorption spectrometry (ARAS / MRAS) or laser absorption spectrometry.
Once the absorption profiles have been measured under the respective boundary conditions, and can have been be converted into concentration profiles by calibration or computation, model designing work begins by developing a matching reaction model for describing the main reaction steps. In this context, the experimental profiles are then compared with the predictions made by the reaction model, and the reaction(s) being studied are adjusted by fitting the rate rate coefficient(s) to the profiles, temperature by temperature and experiment by experiment, until a coherent picture emerges and k = k(T) can be derived.