1830 SPECTROSCOPY AND FLUORESCENCE OF PIGMENTS CHAP. 37C 



Table 37C.V 



Effect of the Wave Length op the Exciting Light on the Quantum Yield of 

 Fluorescence, <f> (after Forster and Livingston 1952) 



This re-absorption, and the consequent reduction in the fraction of absorbed energy 

 that escapes the solution as fluorescent light, may explain, according to a suggestion by 

 Forster and Livingston, how some chlorophyll-sensitized photochemical reactions can 

 have quantum yield close to 1.0 (c/. chapter 18, Vol. I, and chapter 35, sections A5 

 and A6). 



(6) Polarization 



The polarization of chlorophyll fluorescence was first measured by 

 Perrin (1929), in four solvents of different viscosity, from petroleum ether 

 (t; = 0.003) to castor oil (7; = 8.32). The degree of polarization of fluo- 

 rescence, excited by linearly polarized light, ranged in these four solvents 

 from 0.01 to 0.42 with excitation by red light (660 m/x); from 0.000 to 

 0.105 with excitation by violet light (430 m/i), and from 0.000 to 0.22 with 

 excitation by ultraviolet light (380 m/u). These results were used to 

 calculate, by means of simple assumptions concerning the rotational 

 relaxation time, the (actual) mean excitation time of chlorophyll (c/. 

 section 1 of this chapter). The dependence of polarization on wave 

 length of the exciting light indicated that some absorption bands corre- 

 spond to electric dipole oscillations along an axis different from that in 

 which the emission band oscillation occurs. This dependence was studied 

 more systematically by Stupp and Kuhn (1952). Fig. 37C.21 shows their 

 results for chlorophyll a in castor oil. Perrin's three points fit well Stupp 

 and Kuhn's curve. According to Perrin's theory, when the relaxation 

 time is long compared to the excitation life time, the degree of polarization 

 can vary between +0.50 for parallel oscillations in absorption and emis- 

 sion and —0.33 for mutually perpendicular oscillations. Fig. 37C.21 

 indicates that the condition i(relax) ^ t(excAt.) is fulfilled for chlorophyll 



