54 SECTIONAL ADDRESSES. 



the principle of a single quantum of activation, took a further step and 

 assumed that this quantum /(Vj could be absorbed when the reactant 

 molecules in the absence of all catalysts were exposed to radiation of the 

 frequency Vj. They had no justification whatever for this assumption 

 and it is germane to ask why the fact that no substance showed an 

 absorption band at the critical frequency v, was considered to be of no 

 great importance. The phenomena of photoluminescence afford very 

 convincing evidence of the existence of molecules in different states of 

 activation, each with its own critical quantum of activation. They also 

 establish the fact that although this critical quantum can be radiated as 

 phosphorescence, the molecules cannot absorb it at the critical frequency. 

 Although the activated states responsible for phosphorescence are 

 characterised in general by their very long life periods, the fact that the 

 activation cannot be achieved by a simple absorption process may be 

 accepted as a proof of the incorrectness of the assumption made in the 

 second part of the radiation hypothesis. This evidence is independent of 

 the ad hoc criticism by Lindemann and by G. N. Lewis. At the same 

 time the evidence is in favour of the reality of the critical quantum of 

 activation, which is the fundamental tenet of the radiation hypothesis. 

 An enquiry into the possible methods of activation whereby a reactant 

 molecule can gain its critical quantum was made necessary, because the 

 theory of activation by collision has not met with complete success, as 

 no proper relation with photochemical activation has been established. 



Photoactivation of a molecule results from the absorption of a single 

 quantum of energy at a frequency Av^ in the visible or ultra-violet which is 

 specifically characteristic of the molecule in its initial state. This 

 quantum Av^ is invariably larger than the critical quantum of activation 

 /(v,, and this is the explanation of Stokes' law in photoluminescence. The 

 difference between the two quanta is radiated during the activation 

 process as a single quantum of fluorescence, so that 



The same relation has been found to hold in a photochemical reaction and 

 fluorescence is an indication of the formation of an activated state of the 

 molecules. In the absence of fluorescence the expected photochemical 

 reaction does not occur, and it may be deduced from this that the quantum 

 efficiency will approximate to unity (in the absence of the chain 

 mechanism) when fluorescence is fully developed, and very small indeed 

 or zero in the absence -of any measurable fluorescence. The expression, 

 given by W. C. McC. Lewis, for the observed heat of a reaction 



Q_N//(V,-Vj, 



where 7;v,, and /(V^ are the critical quanta of activation of the reactant 

 and resultant molecules, respectively, has been extended to photochemical 

 reactions. In a monomolecular reaction which is photochemical and 

 reversible the observed heat of reaction is given by 



Q=NAK-Vo), 



where Vg and v, are the characteristic ultra-violet frequencies of the 

 reactant and resultant molecules, respectively. 



