INTRODUCTION 



53 



Electron is in nor- 

 mal or ground state 

 at time / = sec 



Electron is in an 

 excited state after 

 absorbing photon 

 of energy E^ — Eq 

 after 1= 10~ 8 sec 



(a) 



Electron has lost 

 some excitation en- 

 ergy — is in lowest 

 allowed excited 

 state 10- n -10- 8 

 sec after reaching 

 Es 



Emitted 

 photon 



Electron is back 

 in ground state 

 after emitting flu- 

 orescence photon 

 of energy E l — E Q 



10- 



10- 



sec 



after reaching E x 



Incident 

 photon 



Emitted 

 photon 



Electron is excited, 

 but not to an 

 energy level of the 

 material, Z«10~ 8 

 sec 



Electron is back 

 in ground state 

 after <<10 -8 sec. 

 Emitted photon 

 has same energy 

 as absorbed photon 



(b) 



Fig. 22. (a) Light absorption and emission if photon energy equals an energy 

 difference in the material, (b) Light absorption and emission if photon energy 

 does not equal an energy difference in the material. 



for the bulk of the light energy absorbed: (a) chemical reactions, (b) 

 fluorescence, (c) phosphorescence, (d) energy transfer, and (e) internal 

 conversion. 



(a) Photochemistry — the study of the chemical reactions resulting 

 from the absorption of light — is among the more fascinating provinces of 

 current biochemical research. One of its most significant aspects is 

 photosynthesis. Since the latter is dealt with in standard introductory 

 biology courses, it will not be covered here. The more esoteric chemical 

 aspects are beyond the scope of these writings. Thus the only segment to 

 be covered in this monograph is that of action spectra, to be dealt with 

 in a later section. 



