1156 



THE LIGHT FACTOR. III. COLOR 



CHAP. 30 



obsei-ved by Emerson and Lewis, and Haxo and Blinks. The latter 

 (1950) observed, in the action spectmm of Ulva (fig. 30.11), a decline 

 in the far red quite similar to that found by Emerson and Lewis with 

 Chlorella. Altogether, the weight of experimental evidence seems to be 

 against the results of Noddack and Eichhoff . This is remarkable, because 

 from the theoretical point of view one would rather expect the quantum 

 yield to remain constant within the red absorption band of chlorophyll. 

 The general experience in photochemistry is that wave length is not im- 

 portant for the photochemical effect, as long as one remains within a single 

 band system, even if this system extends over all colors of the rainbow. 

 The reason is that, within a single band system, the electronic excitation 

 energy is constant, and all excess energy absorbed by the molecule serves 



14 

 12 



10 

 8 

 6 

 4 

 2 

 



70 



60 



- ^- 50 



Q. 40 



q: 



O 



CD 30 



< 



20 



10 



Dense suspension /, . 



15 min. V — 



/ \ \ Assimilation 



\ 1 -^^ Absorption 



^ /••~-,^'^'" suspension \ V'v^ 

 \ / / 30min^ ^ \ \ ^ 



500 600 700 



WAVE LENGTH, m^i 



ROO 



I''ig. 30.1. Action spectrum of Chlorella (after Eichhoff 1939). Scale at left 



represents assimilation. 



merely to increase its vibrational and rotational energy. If the primary 

 photochemical process is the dissociation of the absorbing molecule, the 

 only effect of variations in wave length is that the dissociation products 

 separate with different relative velocities; usually, this excess energy does 

 not affect the ultimate fate of the dissociation products, because it is lost 

 by collisions before the next reaction step. If the primary photochemical 

 process is electronic excitation, the excess vibrational energy acquired by 

 the absorbing molecules also will usually be lost before the occurrence of 

 the secondary reaction. However, exceptions to this behavior are known. 

 In some cases, the electronic excitation energy i.s too small to bring about 

 dissociation without the assistance of a. certain amount of vibrational 

 energy (an example is the delayed monomolecular photodissociation of 

 large molecules, described by Franck and Ilcrzfcld, 1937, and considered 



