ABSOLUTE QUANTIAf YIELDS OF FLUORESCENCE 



109 



cence yield of chlorophyll a and of phycocyanin in solution as a func- 

 tion of the wavelength of the exciting light. For each of the two 

 pigments, the quantum yields were found to be constant (within 

 ±6%) for excitation between 405 m/x and a wavelength slightly 

 beyond the red absorption maximum. The similarity of the results 

 obtained with the two dissimilar pigments and the agreement of 

 these findings with those obtained by Forster and Livingston in the 

 study of chlorophyll fluorescence may be regarded as indirect 

 evidence that there were no serious errors in the calibration curve. 



P3,0r 



o 



^ 20 



> 

 E 



c 

 o 

 a 

 O 





1.0- 



4 Fluorescence observed through Schott RG9 

 • . ... RG5 



o • ■ ■ Corning 2403 

 I I I I I I 1 



4 6 8 10 



Absorption (Loglo/I) 



12 



Fig. 2. Fluorescence of Chlorella 

 cells. Quantum jdeld as function of 

 cell concentration. (Log ly/I is propor- 

 tional to concentration. Path length 

 of exciting beam = 51 mm.) Yields 

 extrapolated linearly to infinite dilu- 

 tion by method of least squares. 

 Average intensity of exciting light = 

 50 ergs/(cm.^ sec). Xex. = '436 m/i. 



350 



40- 



•s 3 



e20 



lO- 



H 



"i I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 



''-lO I 23456 



Log Incident Energy (ergs/cm^ sec ) 



Fig. 3. Fluorescence of Chlorella 

 cells. Fluorescence as function of 

 intensity of exciting beam (averaged 

 over its path in vessel). X„x. = 436 m/*. 



The average sensitivity of the detector to the fluorescence emitted 

 by the different pigments and cells was determined from the calibra- 

 tion curve of the detector system and the known fluorescence spec- 

 tra. 



In order to correct for the reabsorption of fluorescence, the yields 

 of fluorescence of several suspensions (or solutions) of different con- 

 centration were measured, and the results extrapolated to infinite 

 dilution. This method permits an accurate correction for the re- 

 absorption of the fluorescence in solutions^ but it does not account for 

 self-absorption within the emitting cells in cell suspensions. The latter 



