Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

122 EXPERIMENTAL METHODS FOR FAST REACTIONSis passed through the solution and the changes produced are monitored spectrophotometrically’2,either at a known time after the electron pulse with a spectroscopicflash and a spectrograph or continuously at a given wavelength with a steadylight source and photomultiplier (cf. Fig. 2). The same opposing demands of shortduration but high intensity of the radiation pulse apply as for the principal flashin flash photolysis. Also, if the reactions studied are not first order, the absorptionof radiation must be uniform throughout the reaction cell. As in flash photolysiswith solutions, care must be taken to de-gas the sample properly. In thepulse radiolysis of aqueous solutions the last traces of impurity, which can interferesignificantly with the reactions under study, can be removed in rather a simple way.The hydrated electrons produced on irradiation are found to disappear more slowlyafter successive pulses, and it has been postulated that they are reacting with theresidual impurities to give non-reactive products. If this is so, then, after a sufficientlylarge number of pulses, the last traces of reactive impurities should have beenremoved, and the rate of disappearance of the hydrated electrons should reach areproducible value. This is found to be the case, generally after about 20 pulses.Two recent reviews’ 3,z4 include discussions on kinetic determinations in radiolysisexperiments.1.2.1 The hydrated electronCertainly the most exciting applications of pulse radiolysis to date have beenconcerned with the reactions of the aqueous electron, e,;. Although there may bedisagreement as to the mechanism of their formationZ4, it is now accepted thatwhen ionizing radiation (notably an electron pulse) is passed into water, hydratedelectrons are produced. They are always present in comparatively small concentrations(about 10-8-10-6 M) and so in most reactions the other reactant isinexcess;thus first order kinetics are nearly always observed. One property that makes e,; especiallyeasy to study is its strong optical absorption in a region of the spectrumwhere most other species are transparent (A, = 720 mp, E, = 15,800 mole.1-l.cm-’). The half-life of ea; in neutral water is a few tenths of a millisecond.The most important reaction leading to decay under these conditions isea;+ H, 0; + H + H2 0. The hydrated electron is surprisingly unreactive” towardsH,O (k 6 16 mole.1-’. sec-’). This is very fortunate since it allows thereaction with any species to be studied where the rate coefficient is greater than aboutlo5 mole.1-l. sec-l.Besides providing invaluable data for checking present theories of the mechanismsof electron-transfer reactions, ea; is interesting in reacting with an extremelywide range of species. The reactions all follow the same basic schemeea;+AB + AB- + A+B- with a neutral species,