21 



The quantum mechanical selection rules allow no more than t^'o 

 electrons to occupy one energy level and this only if the two 

 electrons have an opposite spin, in which case the two "coupled" 

 spins neutralize one another's outward electromagnetic actions, 

 making the molecule diamagnetic. 



W 



// 



G 



Fig. 5. Schematic representation of an electronic excitation. G: ground 

 level; Si: lowest singlet level, S2: next higher singlet level; Ti: lowest 

 triplet level; T2: next higher triplet level. Vibrational levels are omitted. 

 Short arrows mark forbidden transitions. 



Such fluorescence as that of rhodamin can most conveniently be 

 observed in the dark, the solution being illuminated by a high 

 pressure mercury lamp armed with a light filter which allows only 

 near UV light to pass. The UV excites the dye but does not inter- 

 fere with our obser\^ation, being invisible to the human eye. My 

 observations described in this book have been made with such an 

 illumination. The fluorescent light has a somewhat longer wave- 

 length than corresponds to the absorption spectrum, some of the 

 energy being lost on the way. So while the absorption maximum 

 of rhodamin is at 554 rcifi, the maximum of its fluorescent band is 

 at 578 nifi. 



Such an excitation, in which the electron is simply raised to a 

 higher energy level and then drops back again, is called a "sing- 



