Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

114 EXPERIMENTAL METHODS FOR FAST REACTIONSPulse radiolysis is very similar in principle. It may be regarded as the “radiation”equivalent of flash photolysis, in which a pulse of ionizing radiation replaces thelight flash. Its application so far has been mostly in the field of solution kinetics,principally aqueous reactions, although it has recently been applied to gaseous systemsas well’. It has proved to be an excellent way of following the reactions of theaqueous electron, and many of the most interesting results have come in this field.In Section 1 flash photolysis and pulse radiolysis are discussed together. The applicationof the former method to a couple of systems and of thelatter to some studieson the hydrated electron are also considered.Shock tube experiments have been carried out to a limited extent with condensedphase systems but they have been used primarily to follow the kinetics of very fastreactions in the gas phase. The reactants are contained in a long narrow tube andsubjected to a very sudden, large increase in pressure. This is accompanied by asudden temperature rise, which is sufficient to excite the components and oftencause dissociation, typically into free radicals. The subsequent behaviour of theseradicals can be followed. From the point of view of trying to obtain a completeunderstanding of the elementary reactions, it is important to inquire into the mechanismsof energy relaxation between the various degrees of freedom of the excitedspecies formed. Work along these lines has been done, but in Section 2 thediscussion is limited to the use of shock tubes for following the actual chemicalreactions.One of the principal difficulties in following reactions with half-lives betweenabout and sec, then, is to initiate the reaction in a sufficiently shorttime. The problem is overcome in relaxation techniques by starting with a solutionin which the reactants are already thoroughly mixed and equilibrated. Such a systemmay be used for all the commonly accepted “equilibrium” reactions, but itmay also be used for investigating a stationary state in which there is no net reaction.Under a given set of conditions, for example defined concentrations, temperatureand electric field-strength, the position of the system, which for conveniencewill be referred to as an equilibrium, is defined. If one of the external parameters(e.g. temperature or electric field-strength) is now altered, a new equilibrium statewill generally result. If the perturbation is performed sufficiently faster than thechemical shift can occur, the latter can be observed independently. This type ofdelayed response is called relaxation, and the term can be applied to many effects(e.g. the delay in reorientation of dipoles in a changing electric field). When the delaybetween cause and effect is associated with the finite time taken for one or morechemical components to react, the process is called chemical relaxation. From ananalysis of this behaviour it is possible to determine a characteristic time-the relaxationtime, T-which may be related to the rate coefficients for the reactionsinvolved in the equilibrium and to the equilibrium concentrations of the reactants.In Section 3 the two main categories of chemical elaxation techniques are discussed.They are the transient or step-function methods in which the approach to the new