fi KADIATION HIOLOGY 



oNtT Iroiu OIK' state to llic otlicr lias hccoMK- so j;rcat that the mean life of 

 the excited molecule has been reduced to a value comparable to its period 

 of rotation. 



Some niolcculcs which cxhil)it a fluorescence yield of unity at low pres- 

 sures dissociate when they are illuminated at high pressures. A collision 

 of the excited molecule with a normal molecule of its own kind or of an 

 added gas induces its dissociation. A process of this type, which is called 

 "induced predissociation." was first observed for I2 (lierzberg, 1950). 



INTERNAL CONVERSION 



In addition to fluorescence, direct optical dissociation, and predissocia- 

 tion, excited complex molecules can undergo a process called "internal 

 conversion" (Franck and Sponer, 1949). This process consists in a radia- 

 tionless transition from a low oscillational level of a higher electronic state 

 to a high oscillational level of a lower electronic state. The difference in 

 energy between the two electronic states appears as an increase in the 

 oscillational energy of the molecule after the transition. In internal con- 

 version, both the initial and final states are (juantized; in this respect 

 internal conversion differs from predissociation. Like predissociation, it 

 can occur only when the molecule is in a specific nuclear configuration for 

 which the total energy and the nuclear configuration of the molecule are 

 the same in the two electronic states. Since a complex molecule has 

 many generalized oscillational degrees of freedom, the time required for 

 the molecule to reach the required configuration may be relatively long. 

 Under experimentally realizable conditions, the time between collisions 

 appears to be much less than the average time required for the molecule in 

 the lower electronic state to return to the crossing point and thus to have 

 a chance of coming back to the original state. Collisions between sur- 

 rounding molecules and the vibrationally excited molecule quickly reduce 

 the vibrational energy of the latter and so make the reverse transition 

 impossible. In this way, internal conversion followed l)y a number of col- 

 lisions of the second kind can lead to the complete degradation of the 

 energy of excitation into thermal energy of the system. This is very 

 probably the explanation of why many molecules which absorb strongly 

 in the visible or near ultraviolet are nonfluorescent and do not react 

 photochemically. 



Immediately after the act of internal conversion, the molecule has a 

 large amount of energy in its oscillational degrees of freedom; in other 

 words, it is a " hot " molecule. As such it can undergo pyrolytic reactions 

 such as decarboxylation or the elimination of a hydrogen molecule. It is 

 diflRcult to conceive of any other simple explanation of the direct photo- 

 chemical dissociation of a complex molecule into two stable molecules, a 

 j)rocess which requires the simultaneous breaking of several bonds and the 

 formation of new bonds. Depending on the nature of the molecule and 



