340 PHOTOMECHANICAL CONSIDERATIONS 



If the observed EPR signal truly represents information about the 

 primary conversion process, then the quantum yield of spins should 

 be of the order of unity. The measurement of the quantum yield must 

 await the solution of the difficulties mentioned above. Unless the 

 quantum yield of spins can be observed under conditions in which it is 

 independent of concentration and light intensity, the information ob- 

 tained is misleading. 



LOW TEMPERATURE 



Reduction of the temperature at which signals are studied should 

 allow the exclusion of those processes contributing to the EPR signal 

 which require spatial migration of chemical species. Observations on 

 R. rubrum at low temperature have been reported by Tollin, Sogo and 

 Calvin (16), A reversible photoinduced signal with rapid kinetics was 

 observed. Reversible photoinduced signals at these low temperatures 

 suggest that at least part of the observed room temperature EPR sig- 

 nal results directly from a temperature-independent physical primary 

 quantum conversion act, as postulated in the model. Optical absorption 

 changes in Rhodopseudomonas spheroides at temperatures of 77°Kand 

 1°K have been observed by Arnold and Clayton (17). We are proposing 

 to associate the temperature independent component of the EPR signal 

 with the mechanism producing the changes observed in the optical ab- 

 sorption spectrum by these investigators. 



TRANSIENT RESPONSE TO A LIGHT PULSE 



Preliminary experiments observing the response of the EPR signal 

 when R. rubrum chromatophores are irradiated with a brief pulse of 

 light have been performed. A square light pulse of 1 second duration 

 was used, which had a rise time of =^10 ^sec. The incident light inten- 

 sity was approximately lO^^ quanta/sec- cm2, predominantly in the 

 wavelength region 7000 to 8000 A. 



The EPR signal, i.e., the maximum value in the derivative of the 

 absorption curve, occurred with a one-half rise time of the order of 

 magnitude of 100 milliseconds. The decay time observed varied from 

 several seconds to several hundred milliseconds. This decay time 

 seemed to depend on aging effects in the sample, decreasing with 

 increasing age. 



Through this technique we are endeavoring to determine the kinetics 

 of unpaired spin production by absorbed light and to relate the EPR 

 observation to steps in. or subsequent to, primary quantum conver- 

 sion. 



