Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

3 APPARATUS 63salicylate gives the best film, as it combines high sensitivity to the extreme uvwith low sensitivity to stray long wavelength light. The basic circuit for a photocellis shown in Fig. 49. This may be used for intensities of 0.01 foot-candle or morewhen the galvanometer is a sensitive one. (See refs. 9 and 22c for amplificationprocedures when lower intensities are encountered.)The procedures employed for the determination of the fraction of light absorbedand of quantum yields with these measuring systems are clearly described by Calvertand Pitts’.3.4 RADIOCHEMICAL SYSTEMS (RADIATION CHEMISTRY) (see also Volume 3,Chapter 2)3.4.1 IntroductionAs with photolytic systems radiolysis may be carried out by continuous or pulsedtechniques. The latter will be dealt with in Chapter 2. The elucidation of the mechanismfor a radiochemical system involves determining the extent of ion andhot molecule reactions, leading to the formation of radicals or intramolecularelimination products, and of radical processes. Very often a striking similarityis found between photolytic and radiolytic systems. The value of k1,/kt4 for thereactions (17) and (14)Me- + MeN,Me + CH4+ CH,N,MeMe. +Me. + CzH6(17)(14)in the y-radiolysis of azomethane16’ are in excellent agreement with values obtainedfrom photolytic experiments. Hence, CH4 and C2H6 must be formed bythe reactions of thermalised methyl radicals. The mechanism probably involvesthe initial formation of an electronically excited molecule of azomethane, eitherdirectly or by ionisation followed by neutralisation.Vacuum uv photolysis experiments also have a direct bearing, since ionisationis possible in certain systems. Becker et ~2l.l~~ investigated the photolysis of propyleneat 1470A and 1236A, i.e. above and below the ionisation energy (9.73eV).In spite of the fact that at 1236A 30 % of the absorbed photons result in the formationof C,H;, there is no appreciable difference in the product distributionat these two wavelengths. They conclude that radical production is important inthis system together with the following intramolecular elimination processesReferences pp. 104-111