Many elementary chemical reactions occur on extremely short time scales (down to femtoseconds). Special experimental and mathematical methods are required to study fast processes. Here, we will focus on reactions in solution. Some of the methods described here are also adaptable to gas-phase reactions, but there are also some gas-phase methods that are quite different from anything we would use in solution. Also note that this chapter does not provide a comprehensive survey of methods, even for reactions in solution. The intention is to discuss a few particularly important methods, which hopefully will give you a flavor of the various approaches available.

Flow methods

The continuous flow method

The basic idea behind the continuous flow method is very simple: the reactants are supplied at a constant rate into a tube where the reaction takes place. At constant flow velocity, positions in the tube correspond to specific times since mixing, the relationship between time and the distance from the mixing chamber L being t = L/v. By varying the flow speed or moving a detector along the length of the tube, we can therefore study the course of a fast reaction at our leisure.