R. S. BECKER AND M. KASHA 45 



make it worth while. There is a possibihty that the new paramag- 

 netic resonance absorption methods can be used to detect triplets 

 generally. (2) The phosphorescence lifetime is sensitive to high- 

 atomic-number-atom substitution in a unique way, whereas fluores- 

 cence lifetime is generally insensitive to such substitutions. However, 

 in some simple molecules (e.g., acetone) this distinction is not very 

 clear, since substitution has several effects. It is possible that the new 

 method of environmental perturbations (Kasha, 1952) may resolve 

 such difficulties. (3) The clearest distinction between fluorescence 

 and phosphorescence is the wavelength of the emission, if both emis- 

 sions exist. In a few molecules, only one or the other emissions exists; 

 this is a rare circumstance, but where it occurs, recourse can be made 

 to steps ( 1 ) or ( 2 ) above. However, when both emissions can be 

 observed (for which, of course, rigid glass solutions at low tempera- 

 tures are used; see text), it is always true that phosphorescence is the 

 longer-wavelength emission, whereas fluorescence is the shorter- 

 wavelength emission (see text on "singlet-triplet splits"). Moreover, 

 it can also be said that, if two emissions are found for a molecule, the 

 shorter-lived emission is fluorescence, and the longer-lived is phos- 

 phorescence, regardless of their absolute values. It is virtually inevi- 

 table, spectroscopically, that these values will be quite clearly sepa- 

 rated in magnitude. 



This third distinction will probably serve as the clearest and most 

 valuable answer to Dr. Mason's questions. 



