22 



let excitation" in reference to the single line it generates in tlie 

 spectrum. Howe\'er, there is always a small but definite chance 

 that the excited electron may reverse its spin, which then becomes 

 parallel to that of its earlier partner. This state is called the 

 "triplet state" with regard to the three spectral lines into which 

 the singlet line splits up. There is little probability that this will 

 occur, for quantum mechanical selection rules forbid electronic 

 transitions of different multiplicity, that is transition from singlet 

 to triplet and vice versa. "Forbidden" just means a low probability. 

 But once this transition into the triplet has happened, and the 

 excited electron has reversed its spin, it cannot drop back to its 

 original level to join its earlier partner which is spinning in the 

 same direction. The excited electron is thus trapped in the excited 

 triplet level which, as a rule, has somewhat less energy than the 

 corresponding singlet level, some energy being lost in the process. 

 If the energ)' difference between the two levels is small, heat 

 agitation may raise the electron back to the singlet level (Ti ~> Si, 

 in Fig. 5). If the electron hereby reverses its spin it may drop 

 back to the ground level emitting its excess energy as delayed 

 fluorescence. What mostly happens, eventually, to the electron in 

 the triplet state is that a deactivating heat collision dissipates its 

 excess energy and so the electron disappears from the scene with- 

 out emitting light. The chances of the electron's falling back from 

 the triplet level directly to the singlet level, emitting a photon, are 

 very small and so a light emission, which corresponds to the arrow 

 Ti -^ G in Fig. 5 is a rare occurrence. Such a light emission 

 which corresponds to this direct triplet-singlet transition, dropping 

 from the excited triplet to the ground state is termed now, after 

 the pioneering studies of G. N. Lewis and his associates, p/jos- 

 phorescence, to distinguish it from fluorescence which corresponds 

 to a singlet-singlet transition. Equally rare will be the opposite, a 

 direct transition from the ground state to the excited triplet (G -> 

 Ti in Fig. 5) under absorption of a photon. This absorption is 

 termed triplet absorption. 



