36 LUMINESCENCE SPECTROSCOPY OF MOLECULES 



(approximately 10~'^ M) of the chlorophylls in standard EPA solvent 

 mixture (of. Kasha, 1947) (2 parts ethyl alcohol, 5 parts isopentane, 

 5 parts ethyl ether, measured by volume), at a temperature of 77° K. 

 The chlorophylls were chromatogrammed on a column of packed 

 sugar containing 3% starch, the bands developed by a mixture of 15% 

 (vokime) ethyl ether in petroleum ether. The absorption spectrum of 

 chlorophyll a was spectroscopically free of pheophytin, chlorophyll h, 

 and other plant pigments. The absorption spectrum of the chlorophyll 

 h used indicated approximately 10-15% of chlorophyll a but no other 

 plant pigments. We used ammonia-hypersensitized I-N Eastman 

 Kodak spectroscopic plates. The spectrograph used was a Hilger me- 

 dium glass. Type E 495. 



We are extending our observations to obtain complete vibrational 

 structure of the phosphorescence emission and to obtain a measure 

 of the quantum efficiency and lifetime (decay constant) of the emis- 

 sion. Our exposure time of several hours at a wide slit ( 1 mm ) would 

 seem to indicate that <I>p° is rather small. However, we must point out 

 that until the mean lifetime has been measured, we would have no 

 comparison with the phosphoroscope resolution time quoted above. 

 Only when the mean lifetime is known will it be possible to state with 

 certainty whether our observation was made near the beginning of 

 the phosphorescent decay, or near the tail end of the decay for each 

 excitation cycle. 



(2) EfFect of Electric and Magnetic Fields: Heavy Atom and 

 Paramagnetic Atom Effects in Phosphorescence 



Spectroscopically the mechanism of singlet-triplet transitions is un- 

 derstood in terms of a phenomenon known as spin-orbit interaction. 

 For our purposes we can state that in all molecules there will be a 

 spin-orbit interaction due to the electric fields of the nuclei of the 

 atoms present in the molecule. This spin-orbit interaction may be 

 strongly enhanced by the introduction of high atomic number or para- 

 magnetic atoms into the molecule (cf. Kasha, 1950, and references 

 therein). The effect of such introduction results in the following 

 changes ( 1 ) the fluorescence quantum yield ^f° for the substituted 

 molecule is lowered, (2) conversely, *p° is increased greatly, and (3) 

 the lifetime of the phosphorescence is decreased greatly. Spin-orbit 



