Selection of the Exciting Radiation

The choice of the wavelength is mainly based on the spectral characteristics of the reactant, since it must absorb the exciting radiation thus, the radiation has to correspond to an absorption band of the reactant, possibly close to the band maximum in order to increase the absorption of the exciting light. Moreover, the exciting light must not be absorbed (or, at least, absorbed to a very low extent) by other chemical species, particularly the solvent and the photochemical products. The spectral properties of the most common solvents are reported in detail in [2]. 

Monochromators (gratings and prisms) may be used to select the appropriate wavelength, but solution filters and glass filters are generally preferred, because of practical and economic reasons. Four types of filters are available, that are shown in Fig. 4.2  

Solution filters. They are both short-wavelength cut-off or wide band-pass filters (a and c in Fig. 4.2), mainly constituted by aqueous solutions of inorganic compounds. Several of them, which cover the wavelength interval between 200 and 650 nm, have been described in detail in [2] and references therein. It is however to be noted that some of these filters may undergo thermal or photochemical reactions, so that a periodic control of their absorbance is recommended. 

A combination of band-pass and cut-oflf filters may provide a relatively narrow band-pass filter system having high transmittance at its maximum a compilation of these systems is reported in [4, 5]. 

Solution filters that absorb infrared radiations are also available for example, the 100 g/L aqueous solution of CUSO4 5H2O with a 2 cm optical path. They warm up and so a eooling system must be provided. 

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