1794 



SPECTROSCOPY AND FLUORESCENCE OF PIGMENTS CHAP. 37c 



structure, Avbich can be treated, in first approximation, as a rectangular — 

 rather than circular— potential box (fig. 37C.1C), oscillations || and -L to 

 the long axis of symmetry should have different frequencies and intensities; 

 the corresponding absorption and emission bands should therefore consist 

 of two components, differing in wave length and strength, polarized in two 

 mutually perpendicular directions. 



■=&. 



PORPHIN 

 ( PROTOCHLOROPHYLL) 



B. 



DIHYDROPORPHIN 

 (CHLOROPHYLL) 



RECT- 

 ANGULAR 

 BOX 



APPROXI- 

 MATION 



TETRAHYDROPORPHIN 

 TBACTERIOCHLOROPHYLL) 

 Fig. 37C.1. Porphin ("round field"), dihydroporphin and tetra- 

 hydroporphin ("long field") conjugated ring systems. 



Table 37C.I summarizes the term estimates, made by the two methods, 

 for the two lowest excited terms of porphin, and the four lowest excited 

 terms of tetrahydroporphin, and their attempted correlation with the 

 empirical absorption bands. 



All these bands arise by transfer of the electron in the filled shell which 

 has the highest angular momentum, 4, into the lowest empty shell. The 

 combination of the angular momentum, 5, of this excited electron, with the 

 angular momentum, 4, of its partner left behind after excitation, produces 



