Quantum yields in polychromatic light

Measurement of Quantum Yields in Polychromatic Light Dinitroaniline Herbicides... 

The determination of photochemical quantum yields is not a simple task, and, in some cases, approximations are required. Nevertheless, according to the parameters chosen, various well-known photochemical reactions can be used to measure irradiance, which is an essential quantity in the field of photokinetics. Finally, some selected chemical actinometers will be discussed with respect to their pros and cons and their best areas of application. At the end, special applications of actinometry such as measurements of polychromatic light and high-intensity light sources (lasers) will be described. The overall aim of this chapter is to help the reader to choose the best actinometers out of the numerous examples in the literature and avoid technical mistakes. 

One additional problem with the quantum yield is in the evaluation of the absorbed radiation inside the photocatalytic reactor (Cabrera et al., 1996). This is due to the fact that semiconductor particles are highly reflective (Fox and Dulay, 1993), favoring photon scattering and reflection (Valladares and Bolton, 1993) reducing, as a result, the photons absorbed. An apparent quantum yield can be considered when the absorbed photon rate is not available, providing a minimum value that bounds the actual quantum yields (refer to Chapter VI). While, the quantum yield was meant for monochromatic radiation, both quantum efficiencies and apparent quantum efficiencies should be considered for polychromatic radiation (Cabrera et al., 1994). As well quantum yields and quantum efficiencies are affected by the light intensity reaching the photocatalyst as reported by Pelizzetti et al (1993). 

Experiments. The equipment has already been described in the section concerning true quantum yields. The only difference is that in this work the polychromatic light was used without filters (Table 6.6). 

Quantum yields are defined only for specific wavelengths. They are not applicable to polychromatic light sources, especially in combination with broad absorption of the irradiated systems, and must be substituted by empirically determined irradiation times. Studies of the variation in the quantum efficiencies of various primary photochemical processes in polymers show very little dependence on molecular weight [858, 871]. 

---