128 PHOTOSYNTHESIS 



medium and exposed to polarized exciting light then the 

 fluorescence is found to be polarized. This is because 

 the excited molecules have not appreciably altered their 

 orientations by thermal molecular movement before the 

 light emission. As the viscosity is lowered so the polariza- 

 tion of the fluorescence is diminished. If the molecular 

 dimensions of the pigment are known then the degree of 

 disorientation with time may be estimated, and thus the 

 time of half life of the fluorescent state be deduced. For 

 many substances the half life is of the order of io~^ sec. 

 (compared with the reciprocal frequency of light io~i* sec). 

 With solutions of chlorophyll only a fraction of the 

 absorbed light is emitted as fluorescence (about io%), while 

 in the living plant the fluorescence yield is still smaller, 

 being less than i%. From the point of view of conversion 

 of light into chemical energy the fluorescence represents a 

 total loss. But the investigation of the fluorescent radiation 

 is of great value for it gives a means of measuring the 

 relative amount of Chi*, provided it can be shown or 

 inferred that the fluorescent yield remains unchanged. 



Energy transfer between pigment molecules {in solution) 



With many substances the fluorescent yield is reduced 

 by increasing concentration, even after allowing for self- 

 absorption of the fluorescence resulting from overlap with 

 the absorption spectrum. In addition it has been found that 

 in a viscous solvent the degree of polarization remaining in 

 the fluorescent light also is lessened by increase in concen- 

 tration. But the important point is that the degree of polar- 

 ization begins to fall' off" before the total fluorescence yield is 

 affected. The interpretation of this result is that the energy 

 of the fluorescent state can be transferred as a whole from 

 one pigment molecule to another, so that molecules in 

 different orientation contribute to the emitted light resulting 

 in a decrease in the degree of polarization. The mechanism 

 of the energy transfer between the molecules is, according 

 to Th. Forster (1949), by the process of inductive resonance. 

 It has been shown that energy may be transferred between 

 molecules separated by a distance of the order of 60 A. 



