The production of monochromatic radiation

Most quantitative photochemical studies are carried out using monochromatic radiation, since the wavelength very often affects the quantum yield ( t)) of a reaction and the relative extents of the possible primary processes. Also, the use of monochromatic light allows the accurate determination of ), the intensity of the radiation, and the fraction absorbed by the reacting medium. The use of different monochromatic sources will indicate whether there are any hot radical effects, since the excess energy available will vary. 

An ideal monochromatic source is the laser or uvaser. There is little or no published work on continuous photolysis using these sources although we must be on the threshold of this happening. Porter and Steinfeld have flash-photolysed phthalocyanine in the vapour phase using a Q-switched ruby laser. This emits at 6943A, and 2-3 J are dissipated in 20 psec. Photolysis occurs because of a two-photon process. 

The unique properties of lasers are their coherence, monochromaticity and high intensity. However, as yet, the intensities of commercial lasers emitting radiation below 6000 A are very low. Their photochemical potential is reviewed by Turro . An excellent account of the theoretical background has been given by Smith and Sorokin . Also, Levine has started a useful series on lasers. Both of these reviews, however, are concerned solely with the spectroscopic potential of lasers. In another connection, stimulated emission from vibrationally excited HCl produced in reaction (16) 

APPROXIMATE WAVELENGTH LIMITS FOR TRANSMISSION OF VARIOUS OPTICAL MATERIALS AND WATER NEAR ROOM TEMPERATURE 

The best way of isolating monochromatic radiation from a source emitting several lines is by means of a prism or diffraction grating monochromator. This 

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