MEUIN CALVIN 347 



chutioii in the Rliodospirilluin appears, namely, one in which the 

 niaxiniuni lor tlie j)ro(luclion of the signal is at somewhat longer 

 wavelength (9100 A) than the maximum lor the pigment absorption 

 (8800 A) (59). 



Part of this shift of the ^v•avelength for most efficient spin signal 

 production is certainly clue to the way in which the experiment was 

 performed. The samples were totally-absorbing and relatively thick 

 (~ 0.1 mm). This resulted in the total absorption of the maximally 

 absorbed light (6800 A for the chloroplasts and 8800 A for the 

 Rhodospirillum) in a very thin layer near the surface of the sample. 

 This situation resulted in the production of unpaired spins at a 

 very much higher real concentration than would be the case if the 

 light were absorbed throughout the sample. This latter situation 

 would be approached by light of wavelengths not so strongly ab- 

 sorbing, such as wavelengths on either side of the absorption maximum 

 in both cases (6800 A for the chloroplasts and 8800 A for the 

 Rhodospirillum) . 



Since there is an indication that the decay rate of the spin sig- 

 nals is greater the higher their concentration, it is easy to see that any 

 attempt to measure the number of spin signals produced for a con- 

 stant incident number of quanta will be in error on the side of too 

 few^ electrons per quantiun absorbed, the greater is the concentration 

 of the unpaired spins produced. This would depress the apparent 

 nimiber of spin signals produced at the very point of maximum light 

 absorption, as indeed is the case. This, however, is not enough to 

 account for the fact that the efficiency of spin signal production is 

 actually higher on the longer side of the maximum light absorption 

 than it is on the short side of the maximum. In order to account 

 for this, another process must be invoked. 



Something else besides the simple absorption of light by the chloro- 

 phyll into its ordinary excited state at 6800 A is involved in the pro- 

 duction of the spin signal. Presumably there is another state, or 

 another substance, which leads to the maximum at longer wave- 

 lengths. In the crystal spectra of the monolayers of chlorophyll (see 

 Fig. 9) there was indeed in the crystalline layers a peak at 7180 A. 

 This, together with the fact that our spin signal occurred at ~7200 

 A, prompted us to seek some evidence for another excited state in 

 the living organism, somewhere around 7200 A. 



In looking back over our earlier studies on the luminescence of 

 living organisms (68, 69), we did indeed find emission at 7200 A. 

 In Fig. 27 is shown the emission spectrum of Chlorella as it has been 



