Visible light sources

In the laboratory it is more convenient to use light either visible or ultraviolet as the source of energy to initiate the reaction Reactions that occur when light energy IS absorbed by a molecule are called photochemical reactions Photochemical techniques permit the reaction of alkanes with chlorine to be performed at room temperature... 

For the visible and near-ultraviolet portions of the spectmm, tunable dye lasers have commonly been used as the light source, although they are being replaced in many appHcation by tunable soHd-state lasers, eg, titanium-doped sapphire. Optical parametric oscillators are also developing as useful spectroscopic sources. In the infrared, tunable laser semiconductor diodes have been employed. The tunable diode lasers which contain lead salts have been employed for remote monitoring of poUutant species. Needs for infrared spectroscopy provide an impetus for continued development of tunable infrared lasers (see Infrared technology and RAMAN spectroscopy). 

Fig. 10-5. Comparison of for 0.1 ppm NO2 and Rayleigh scattering by air. The photopic eye response represents the range of wavelengths over which the eye detects light. Source Husar, R., White, W. H., Paterson, D. E., and Trijonis, J., "Visibility Impairment in the Atmosphere," Draft report prepared for the U.S. Environmental Protection Agency under Contract No. 68022515, Task Order No. 28.

Fig. 10-8. Single particle scattering to mass ratio for particles of four different compositions. Carbon particles are also very efficient absorbers of light. Source U.S. Environmental Protection Agency, "Protecting Visibility," EPA-450/5-79-008, Office of Air Quality Planning Standards, Research Triangle Park, NC, 1979.

Clearly, unless monomer is the intended photoinitiator, it is important to choose an initiator that absorbs in a region of the UV-visible spectrum clear from the absorptions of monomer and other components of the polymerization medium. Ideally, one should choose a monochromatic light source that, is specific for the chromophorc of the photoinitiator or photosensitizer. It is also important in many experiments that the total amount of light absorbed by the sample is small. Otherwise the rate of initiation will vary with the depth of light penetration into the sample. 

Radiation of a particular wavelength (monochromatic) is required to initiate a specific electronic transition, but most UV and visible light sources are polychromatic. For example, common mercury arc lamps emit around 50% of their energy in the 405 nm to 578 nm range. Hence, for most processes, well over half of the electrical energy... 

Raman spectroscopy detects the scattering of light, not its absorption. Superposed on the frequency of the scattered light are the frequencies of the molecular vibrations. The detection occurs in the IR spectral region while the excitation happens in the visible region. Since laser light sources have become well developed, Raman spectroscopy has become an important tool for the analysis of biomolecules. 

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