78 



KASHA: It is not much longer than a vibration period; that has been proven 

 quantitatively. 



BURTON: Internal conversion may be the precursor either of decomposition 

 or of a simple energy degradation, for example by fluorescence. If decomposi- 

 tion occurs, the total process is called predissocitaion. Such predissociation 

 processes, require anything from 10"^ seconds up. A similar statement ap- 

 plies, of course, to internal conversion. The time requirement depends on the 

 necessary process and the probability of that process. There are two kinds of 

 internal conversion: induced and the spontaneous. Induced internal conversion 

 is more probable in a condensed system than in a gaseous system. 



FANO: Wouldn't it perhaps be possible to say that what this lO"" 1 ^ seconds 

 really represents is the time required for the atoms to get out of the position 

 from which they can drop down, so that direct fluorescence will be forbidden by 

 the Franck-Condon principle, but it does not imply there has been within the 

 10" 13 seconds anything like substantial transfer of energy to vibration? 



LINSCHITZ: I would agree with that. Certainly several vibrations are nec- 

 essary before appreciable energy can be bled away from the molecule by colli- 

 sions. However, you only need one vibration period to excite the full amplitudes 

 in a particular mode. 



The question was asked how rapidly these processes occur. Of course, 

 once a moleculesis in an excited state possible processes may occur which will 

 stop the fluorescence. Conversion into a metastable tautomeric state, which 

 has never been clearly shown to exist, would also make this a short lifetime but 

 even conversion to a tautomer requires that the molecule be trapped with loss of 

 vibration energy, somewhere, in order to have any effect on the fluorescence. 

 That trapping process essentially consists of the loss of a certain amount of vi- 

 bration in the condensed phase, since this is the only thing that makes the tau- 

 tomer metastables. So that if you are talking about processes that will lead to 

 heating, these may occur in very short times -- processes that will lead to 

 heating subsequent to excitation. 



FANO: To some heating. 



LINSCHITZ: To some heating, that is right. No matter how far down you 

 drop. 



KASHA: It might be only half a volt in general. Only a small fraction of the 

 electronic energy is degraded in any single internal conversion among other 

 states. 



BURTON: Do you say the electronic energy is degraded during the internal 

 conversion process or subsequent to it? 



KASHA: Well, you start with 6 volts of pure electronic energy and you con- 

 vert it to 5 volts of electronic energy. Incidentally, I disagree with Dr. Burton 

 about one thing. In diatomic molecules what he says is certainly true, but I 

 think the real point of Dr. Linschitz's talk is that for the polyatomic molecule 

 the time of conversion is not even 10"^ seconds. It has to be closer to 10" 12 or 

 lO" 1 ^ because otherwise you would observe emission from these upper states. 



BURTON: Is this a liquid you are talking about? 



KASHA: In liquid or vapor at moderate pressures. 



