Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

118 EXPERIMENTAL METHODS FOR FAST REACTIONSdiscussed in this chapter; it is considered in Section 9. Also mentioned briefly isthe application of molecular beams to the study of fast gas-surface reactions,It may be apparent from this brief introductory survey that each method has itslimitations, and that for a given system probably only one or two would be appropriate.The restrictions are of two types. Some are inherent in the method itself(e.g., a chemical relaxation method requires a system comparatively near to equilibriumor, rather, one in which the equilibrium or stationary state may be easilydisplaced) while others are practical difficulties (e.g., the very limited chemicalrange of the detectors in molecular bean experiments). Most of the methods describedhere are being used in many laboratories and are consequently undergoingcontinuous development. It can thus be confidently predicted that many of thepractical difficulties in applying them to specific chemical systems will be overcomein the near future. Because of this continual improvement in technique not muchhas been said about the range of rate coefficients for which a given method is appropriate,or the accuracy with which the coefficients can be determined. The maximumsensitivity of most methods is within the 10 % level, although the actualsensitivity in a given application depends on several factors associated with the specificsystem, such as the optical absorptivity. Several of the techniques consideredhere have been discussed in one or more of the recent reviews on fast kinetics9-16.1. Flash pbotolysis and pulse radiolysis1.1 FLASH PHOTOLYSISA schematic diagram for flash phot~lysis'~*'~shown in Fig. 1. The detailedarrangements and dimensions vary considerably, but typically the reaction mixtureis contained in a quartz vessel which has a length of 20 cm and a diameter of 2 cm.It is important that the ends are optically flat and parallel, since analyzing light ispassed through the vessel along its axis. Parallel to the reaction vessel is a flash-lighttube containing a rare gas, often krypton; the electrodes of the flash tube are connectedto a bank of condensers. The reaction is initiated photochemically by dis-Magnesiumoxide coatedref lector- - - - - - - - - -Fig. 1. Schematic arrangement for flash photolysis.