Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

3 APPARATUS 553.3.4 The production of "monochromatic" radiationMost quantitative photochemical studies are carried out using "monochromatic"radiation, since the wavelength very often affects the quantum yield (4) ofa reaction and the relative extents of the possible primary processes. Also, the useof monochromatic light allows the accurate determination of c$, the intensity ofthe radiation, and the fraction absorbed by the reacting medium. The use of differentmonochromatic sources will indicate whether there are any hot radicaleffects, since the excess energy available will vary.(a) LasersAn ideal monochromatic source is the laser or uvaser. There is little or no publishedwork on continuous photolysis using these sources although we must beon the threshold of this happening. Porter and Steinfeldl 48 have flash-photolysedphthalocyanine in the vapour phase using a Q-switched ruby laser. This emits at6943A, and 2-3 J are dissipated in 20 psec. Photolysis occurs because of a twophotonprocess.The unique properties of lasers are their coherence, monochromaticity and highintensity. However, as yet, the intensities of commercial lasers emitting radiationbelow 6000 A are very low. Their photochemical potential is reviewed by T ~rro'~~.An excellent account of the theoretical background has been given by Smith andS~rokin'~'. Also, Le~ine'~' has started a useful series on lasers. Both of thesereviews, however, are concerned solely with the spectroscopic potential of lasers.In another connection, stimulated emission from vibrationally excited HCl producedin reaction (16)H.+cI,-+Hc~*+H. (16)References pp. 104-111Fig. 44. A crystal quartz monochromator. See Calvert and Pitts' p. 725.