8 RADIATION HIOLOGY 



ings, and its oscillational enerf^y will Ix' at or near its zero-point value. As 

 determined l»\ the Franck-Condon principle, the electronically excited 

 molecuh^ will usually have an excess of oscillational eiierj^y. In a con- 

 densed medium or in a gas at moderate pressure, the molecule will (|uickly 

 lose this excess of vibrational energy by successive collisions of the second 

 kind. As a result the fluorescent light, transition 3, will usiuilly have a 

 larger wave length than the corresponding absorption. In addition to 

 these permitted transitions, radiationless transitions, 4, 5, and (>, are pos- 

 sible. Each of these acts corresponds to a process of internal conversion. 

 The occurrence of step 4 is proved by the fact that fluorescence cor- 

 responding to transitions from /'" to A'^ or from F' to /'' is never observed 



Fk',. 1-2. Schematic potential-energy diagram for a complex molecule. 



with complex molecules such as dyes. Illumination with light of shorter 

 wave length, which raises the molecule to the second (or a higher excited) 

 level F', results only in the long-w'ave-length fluorescence (transition 3). 

 Phosphorescence and long-lived fluorescence involve the metastable state 

 M . In order to reach this state the molecule must undergo an act of 

 internal conversion, step 5. Once in state M , the molecule can return to 

 state F, by way of step 5, only if it receives thermal energy e(|ual to or 

 greater than e, the difference between the energies of levels M and F. 

 This sequence of events corresponds to the temperature-dependent phos- 

 phorescence. Step 8 is forbidden, by the intercombination rule, and will 

 occur only if there is no other probable mode of escape from M. The 

 relativ^e importance of this emission, the long-lived fluorescence, increases 

 as the temperature is lowered. Internal-conversion steps, 6 or 7, con- 

 tribute to the inefficiency of photochemical and fluorescence processes, 

 (^ne offect of adsorbing the molecule or dissol\iiig it in a rigid medium is to 



