Light Sources Used in Photochemistry

Lasers are particularly important in photochemical research because their stimulated emission produces light which is monochromatic, coherent and very intense. Additionally, advances in laser technology have enabled lasers to deliver pulses of shorter and shorter duration such that today it is possible to produce pulses of the order of femtoseconds (lfs = 10 15s). 

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The mercury lamp has been the conventional light source used in photochemistry. The ground-state mercury atom, Hg, has two electrons in its highest occupied orbital, the 6s atomic orbital. Excited mercury... 

The study of the chemical effects produced by UV or visible light is known as photochemistry. In photochemistry, the absorption of a photon causes excitation of the reacting molecule to a definite, usually known excited electronic state. The principal difference between radiation chemistry and photochemistry is that the energy of the ionization radiation used in radiation chemistry is much greater than that of the light source used in photochemistry. 

Light sources pose a difficulty on an industrial scale. Lamps used in photochemistry include medium- and high-pressure mercury lamps, xenon lamps and halogen lamps, all of which are costly to run. These have a limited lifetime and additionally tend to generate a large amount of heat and therefore require additional cooling systems. 

Abstract In this chapter we discuss some of the typical materials used in photochemistry. We describe, in general terms, how their suitability for application as absorber, emitter, sensitiser, energy acceptor or quencher, depends on the energy states within the material and the routes of interconversion between these states, and also how suitability as a redox or chemical sensitiser/acceptor/ trap is determined by specific chemical reactivities. We describe the application of photochemical principles to the design of light sources and displays, and describe the photochemical principles and applications of photochromies and molecular switches. A table giving the structures, characteristics, and uses, of a number of compounds widely used in photochemistry is provided at the end of the chapter. 

Use of coherent light sources in industrial appHcations has led to the field of photodynamic therapy as a photochemically based medical technology (9—11). The apphcation of photochemistry to information storage and communication processes is expected (12) (see Information storage materials Resist materials). 

Generally the first thing to be done in preparation for the photochemical study of a compound is to determine the visible and ultraviolet absorption spectrum of the compound. Besides furnishing information concerning the nature of the excited state potentially involved in the photochemistry (see Section 1.4), the absorption spectrum furnishes information of a more applied nature as to the wavelength range in which the material absorbs and its molar absorptivity e. From this information it is possible to decide what type of light source to use for the irradiation, what solvents can be used to... 

In the introduction to Volume 1 of this series, the founding editors, J. N. Pitts, G. S. Hammond and W. A. Noyes, Jr. noted developments in a brief span of prior years that were important for progress in photochemistry flash photolysis, nuclear magnetic resonance, and electron spin resonance. A quarter of a century later, in Volume 14 (1988), the editors noted that since then two developments had been of prime significance the emergence of the laser from an esoteric possibility to an important light source, and the evolution of computers to microcomputers in common laboratory use of data acquisition. These developments strongly influenced research on the dynamic behavior of the excited state and other transients. 

Since in industrial photochemistry mostly polychromatic light sources are used, photon quantities are relatively difficult to calculate and require knowledge of the spectral distribution of the radiometric quantity measured. Assuming on the other hand that the radiometric measurements do not need to be corrected for the spectral response of the probe, the photon irradiance at a given point within the reactor volume would then be given by Eqs. (39) and (40), respectively. 

Finally, enrichment of isotopic species has been achieved for a number of atoms and molecules using an appropriate monochromatic light source that preferentially excites an isotopic species of interest in mixtures of other isotopic species. The photochemistry associated with isotopic enrichment is briefly described in Chapter VIII. Great efforts have been made recently to obtain information on the detailed photochemical processes involving smog formation, stratospheric pollution, and atmospheres of other planets, and brief discussions of these subjects are also presented in the chapter. 

A commercial 3 kW laser of this type (60 cm long, vertically mounted) has been used to build a falling film reactor capable of converting 10 g or more in 10-20 h [7]. At least at present, however, these light sources are rather expensive and require considerable care for their maintenance consequently, they cannot be considered for adoption by an organic photochemistry laboratory requiring a versatile tool for preparative applications. 

The photochemistry of triphenylmethyl radicals in solution has been of interest for some time because the relatively long lifetimes of these species (some are essentially stable) have made their study accessible with low-intensity light sources. Relatively recently, laser flash photolysis has been used to confirm the tendency of the excited radicals to undergo ring closure to form the 9-phenyldihydro-fluorenyl radicals. 

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