A couple of years after the Oxford meeting, Taylor pointed out that the quantum theory of absorption and emission of radiation on one hand “amplifies the first law by defining more particularly the absorption act,” but in so far as it “sets a limit to the energy available in a single absorption act, it also sets a limit to the nature of chemical change that can result from such absorption. Thus, for example, if the absorbing system is composed of a single type of diatomic molecules and the light
2.8 The “Laws” of Photochemistry 35
energy absorbed per quantum is less than the energy of dissociation of such molecules, it follows that such dissociation cannot occur as the result of absorption of a single quantum.”
Taylor contrasted the most uncompromising form of this law, where an exact equivalence between absorbed quanta and reacting molecules was postulated, with experimental evidence that showed that this was only exceptionally the case. After examining the available data [75,76], he suggested that “the difficulties inherent in the acceptance of the law of photochemical equivalence as originally formulated would embody the elements of this law which have found support from its study.
For this purpose it seems necessary first to avoid entirely the name which has become usual in reference to this matter, since equivalence has been demonstrated only in exceptional cases, rather than as a general rule.”
“He concluded that the situation may be met by means of two laws of photo- chemistry. The first law of photochemistry would be the Grotthuss–Draper absorp- tion law, embodied in the statement that:
• Only light that is absorbed is effective in producing chemical change.
This would be followed by the second law of photochemistry which might thus be expressed as:
• The absorption of light is a quantum process involving one quantum per absorb- ing molecule (or atom). The photochemical yield is determined by the thermal reactions of the system produced by the light absorption.
Of this second law, the quantum concept of absorption is Einstein’s contribution to the progress of photochemistry. The second half is a generalization from the experiments of numerous workers, who, in testing Einstein’s original ideas, have added enormously to the quantitative knowledge of mechanism in photochemical processes and demonstrated the factors which determine the yield from a given illuminated system”. Essentially, this is the view that has prevailed and the form laws have taken [77–81].
In his talk in Oxford, Allmand had listedca60 quantum yield measurements in 1926. In 1938, Daniels was able to publish a collection of some 240 measurements and proposed that the photodecomposition of uranyl oxalate that had been investi- gated in detail was adopted as universal reference.
He stated that “in the early development of quantitative photochemistry it was believed by some that the Einstein relation would apply in many cases not only to the primary process of photoexcitations but to the overall reaction as well. Quantum yields were summarized with the purpose of testing this hypothesis. Any hope of simplicity in chemical kinetics disappeared long ago, and the present table has been assembled not to emphasize the almost universal occurrence of secondary effects which follow the primary process of quantum absorption, but to record the exper- imental facts of photochemistry in the simplest possible manner. The primary excitation is usually followed by rearrangements and degradation of the energy as heat, by reverse or competing reactions which make the overall quantum yield less than unity, or by continuing reactions which produce a chain and give a value
36 2 The Framework of Photochemistry: The Laws
greater than unity. Sometimes it is possible to study these factors from the magni- tude of the quantum yield and its response to influences such as temperature, wave length, concentration, and chemical reagents. The amount of chemical reaction produced by the absorption of radiation will change with the duration of exposure, the intensity of the light, the thickness of the optical path, the physical condition of the absorbing material, and other factors. The fundamental simple relation between light and chemical action, however, is the quantum yield Φ, i.e. the number of molecules of substance reacting for each quantum of radiation, or photon, absorbed.
When this is known the extent of the chemical reaction produced by the absorption of a given amount of light is easily calculated.”
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