Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

~~ ~36 EXPERIMENTAL METHODS FOR SLOW REACTIONSof radiation the source should be as close to the RV as is practicably possible andall windows and lenses should be perpendicular to the path of the radiation. Alignmentmay be checked roughly using a tungsten lamp or more precisely using afilter soaked with a fluorescent material such as anthra~ene"~. Full details forthe construction of RV's and the sealing of windows to the cells are given in refs. 9and 22c.3.3.3 Sources of radiationCare should be taken to prevent undue (or indeed any) absorption of radiationby the operator, especially by the eyes and particularly with laser sources. Therange of wavelengths of radiation for photochemical reactions extends from 1200 Ato 7000 A. When studying photochemical reactions it is particularly useful to beable to convert wavelength (A) in A to energy (E)in kcal. Einstein-'. This is givenby the expressionE = 2.8591 x 105/h kcal. Einstein-'(B')The wavelength range is divided into several sections which are shown in Table 4.As far as this discussion of sources is concerned, the range is divided into two;the vacuum uv region is treated separately.TABLE 4THE PHOTOCHEMICAL RANGEA Region Energy(kcal. Einstein-')12002000300040007000Vacuum uvFar uvNear uvVisible238143957141(a) Visible to fur ultra-violetMercury lampsThe most common sources of radiation for kinetic photochemical studies aremercury lamps of which there are three types. The low-pressure lamp is usedmainly for mercury-sensitised studies. For general photochemical studies the mostuseful lamp is the medium-pressure lamp having several "lines" of reasonable