Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

1 FLASH PhOTOLYSIS AND PULSE RADIOLYSIS 121shown that the antiknock effect of tetraethyl lead in this particular system occursentirely in the gas phase.(2) A system which is important not only because of its intrinsic interest but alsobecause it lies at the very heart of life is photosynthesis. Flash photolysis is an idealtechnique by which to measure the effect of light on the various components ofchloroplasts, both in solution and as suspensions of the chloroplasts themselves.In fact it has been necessary to pushthe technique towards its limits both as regardssensitivity of absorption and also time measurement. This topic has been discussedby Witt et aL2'. The conventional technique has measured transmission changes of0.01 % in times as short as sec, but a method known as periodic flash photometryhas increased the sensitivity considerably. In conventional flash photolysisthe whole sequence of absorption changes following a flash is recorded. But if ameasurement can be made several times its accuracy increases with the square rootof the number of readings. Use can be made of this by storing many traces of thecomplete reaction in a computer of average transients (CAT) (optical density changesas small as 0.0001 have been measured at about 50 psec after the flash2'). Alternatively,the complete reaction sequence may be split up into small time-segments(e.g., by the use of choppers in conjunction with the flashes), and the absorptionof the system during a given segment measured many times. The absorption ofthe next segment is then measured, and so on, until the entire course of the reactionhas been covered.1.2 PULSE RADIOLYSISThe main differences between pulse radiolysis and flash photolysis arise from theuse in the former of ionizing radiation instead of light to initiate the reaction. Thusa pulse of electrons from a linear accelerator (or, less commonly, an X-ray pulse)Electron source IReferences pp. 176-1 79Light detector- NM3Fig. 2. Schematic arrangement for pulse radiolysis. MI-M, are mirrors.