REMARKS ON THE SIGNIFICANCE 

 OF ACTION SPECTRA 



FREDERICK S. BRACKETT and ALEXANDER HOLLAENDER i 



National Institute of Arthritis and Metabolic Diseases, National 

 Institutes of Health, Bethesda, Maryland 



Action spectra are often obtained as an aid in identifying the molecules 

 initiating an action through comparison with known absorption 

 spectra. Unfortunately, other factors besides absorption may contribute 

 to the form of the action spectrum. These factors include: (1) quan- 

 tum efficiency, (2) screening by overlying layers, and (3) competi- 

 tion with other absorbing molecules. 



Most dramatic is the case where the quantum threshold lies within 

 that portion of the spectrum where absorption exists. At the threshold 

 the action rises sharply from zero on the long wavelength side to a 

 value controlled by the quantum efficiency on the short wavelength 

 side. The quantum efficiency may be a fraction, often near 1, but 

 sometimes a large value as in case of a chain reaction. 



Let us consider a case where n quanta are required to produce a unit 

 action (molecule transformed, cell killed 50% of time, etc.), 1/n being 

 the quantum efficiency. Then Nn quanta are required to transform N units 

 per unit volume. With In radiant power incident per unit area acting for 

 time / with a fraction A absorbed, in x thickness, the energy absorbed per 

 unit volume in time / is htA/x in customary units. Owing to screening and 

 competition only a fraction ^ may be available, so 



Nn=^<i>^-^ (1) 



xhv 



In an action spectrum one usually plots N 'I„t for the chosen end point, so 



^=A^ (2) 



IqI nhv X 



It would of course be preferable to express the action in terms of quanta 



1 Present Address: Oak Ridge National Laboratory, Oak Ridge, Tennessee. 



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