Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

3 APPARATUS 35Apart from the two fundamental laws of photochemistry, a very important relationshipfor the experimentalist is the combined Beer-Lambert law. This describesthe extent of absorption of monochromatic light by a homogeneous system, andis given by equation (A')log I,/I = ECI(A')Here I, is the intensity of incident monochromatic radiation, I is the intensity ofradiation at a distance I cm, and E is the decadic molar extinction coefficient of anabsorbing species (concentration, c mole. I-'). This law is strictly valid only ifmolecular interactions are unimportant at all concentrations. Deviations occur for avariety of reasons; this means that the validity of the law should be checked underthe particular experimental conditions. An initial determination of the absorptionspectrum of the compound under investigation is obligatory. This produces immediatequalitative information, particularly about the usefulness of the source ofradiation. Banded, diffuse or continuous spectra give direct information about thecomplexity and variety of primary processes that may occur. Further informationwill be gained from the effect of radical traps such as 0, or NO, and of variousenergy transfer agents.3.3.2 The optical set-upA typical optical system is shown in Fig. 26. A lens of short focal length (7-10cm) projects a nearly parallel beam of radiation from the source A through a filterF, to remove unwanted radiation. The stop S, prevents unfiltered radiation fromreaching the RV. It is sometimes useful to converge the beam slightly with a secondlens (focal length -40 cm) such that the beam reaches its smallest diameterin the centre of the RV. The latter may be divided into two compartments, one ofwhich contains a compound used for actinometry, or alternatively the beam isfocused by the lens L, onto a photocell or thermopile P. The intensity of the beamis suitably reduced by the density filter F,. To provide maximum possible intensityReferences pp. 104-1 11Fig. 26. Optical set-up for photochemical experiments. From ref. 9.