Heterogeneous Systems Photonic Efficiencies

With some caveats apparent later, another parameter that may characterise the photocatalytic activity of a heterogeneous system is the photonic efficiency (, eq. 5.87 Serpone et al, 1993b), which expresses the ratio of the rate of molecules... 

In other words, photonic efficiency is a reaction rate normalised to the photon flow emitted by the light source. It describes how many molecules are transformed per photon incident on the system. Of course a more appropriate parameter to characterise the photochemical activity of heterogeneous systems is the more fundamental quantum yield (, eq. 5.88) of the photoreaction, which expresses (Serpone and Emeline, 2002) the ratio between the rate of molecules formed or degraded in the system, dNJdt (molecules s ), and the rate of photons absorbed by the system. 

To establish both the photonic efficiency and the qnantnm yield of a photo-stimulated reaction in heterogeneous systems, two important conditions (Emeline et al, 1998a, 2000c), which unfortunately many researchers fail to consider, must be satisfied (i) the reaction rate must scale linearly with photon flow p, and (ii) the reaction rate must be independent of the concentration [M] of reagent molecules. Otherwise, where both photonic efficiency and quantum yield depend on photon flow and reagent concentration, the photocatalytic activities described by and (j) from different heterogeneous systems and different laboratories cannot be compared. 

On the basis of the above considerations, we now describe briefly the protocol(s) to assess the efficiencies of the photocatalytic process(es) (Salinaro et al., 1999 Serpone and Salinaro, 1999). It will be necessary first to determine the photonic efficiency of the heterogeneous process as expressed by eq. 5.87, remembering that the absolute values of have little meaning since they are obtained for photons incident on, and not absorbed by, the reacting system. Nonetheless, they do provide a point of departure. 

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