70 LIGHT AND LIFE 



available from (1) and (2) . It gives directly the energy of higher 

 triplet levels. But much more important is the fact that it makes 

 possible the direct measurement of triplet concentration as a function 

 of time and is equally applicable to gases, liquids, and solids. In this 

 symposium the interest is principally in solutions and the remainder 

 of this paper will deal only with this medium. 



Flash Photolysis 



Since the lifetime of the triplet state in fluid solution is normally 

 not greater than one-thousandth of a second, special experimental 

 techniques must be used. The technique of flash photolysis, originally 

 developed for the study of free radicals (7) , is ideally suited to this 

 purpose and is indeed the only general method at present available 

 by which the triplet states of molecules can be observed and quanti- 

 tatively studied in fluid solution. 



A flash photolysis apparatus comprises a reaction vessel of some 

 optically transparent material, an electronic flash which delivers a 

 flash of light of at least several hundred joules energy and of a few 

 microseconds duration, and some means of recording the transient 

 absorption spectra of the reaction mixture as a function of time after 

 the flash. This recording may be carried out either by means of a 

 second electronic flash timed to photograph the absorption spectra 

 at preset time intervals after the photolysis flash or by the continuous 

 photoelectric recording of optical density at one wavelength. The 

 former method is most suitable for the initial study of the transient 

 spectra, and the latter method is more convenient and accurate for 

 detailed kinetic studies once the spectrum is known. 



The first application of flash photolysis to the triplet state and 

 the first detection of triplet state absorption spectra in solution were 

 made by Porter and Windsor in 1952 (10). At this time it was not 

 entirely clear whether the general absence of phosphorescence in 

 gases and solutions was due to the fact that the conversion from upper 

 singlet to triplet state occurred less readily than in solids or to the 

 existence of radiationless processes in solution which were much more 

 rapid than phosphorescence. Experiment showed that the latter was 

 the case and that trij)lct states were readily populated in fluid solvents 

 but normally had a lifetime of the order of 10-* seconds. This time 

 was short enough to account for the absence of long-lived jihos- 

 l)horescence l)ut long enough to be readily studied by flash photolysis 

 techniques. Absorption spectra of a wide variety of molecules were 

 recorded, and preliminary kinetic investigations were carried out by 



