20 



system of conjugated double bonds. One of the major group* a 

 made up of aromatic hydrocarbons, which have no charges and are 

 unable to accept or give off protons. Accordingly, their fluorescence 

 is independent of pH and redox potential. Their absorption lies 

 mostly in the UV. 



Another important group of fluorescent substances is formed by 

 molecules which have charges, the oscillations of which partake in 

 excitation. The fluorescence of these molecules is sensitive to pH 



(C2H5)2N .,,,^^-^0.^^^^ N (C2H5)2CI 



XOOH 



Fig. 4. RJiodamin B. 



and redox potential; their absorption is mostly at a longer wave- 

 length, in the visible part of the spectrum. Dyestuflts belong to 

 this group. One of the classic members of this group is the dye 

 rhodamin B (Fig. 4), the charge of which oscillates between its 

 two nitrogens. Rhodamin has an absorption band in the UV and 

 one in the visible with a maximum at 554 rrifi. So if irradiated by 

 UV it will absorb this light, but according to laws of spectroscopy, 

 it will shoot out its fluorescent light from the lower absorption 

 band. This is symbolized in Fig. 5, in which the line G stands for 

 the normal or ground state of the molecule. The arrow G -^ S2 

 stands for the excitation of the molecule by the UV light. The 

 arrow S2 -> Si stands for the "internal conversion" in which the 

 electron gives off in a photon-free process its excess energy and 

 drops to the energy level of its lower absorption band, the energy 

 difference being dissipated as heat. The arrow Si -> G symbolizes 

 the shooting out of the energy in the form of the orange fluores- 

 cent light, whereby the electron drops back to the ground level, 

 joining its pair. As is generally known, electrons, as a rule, occupy 

 orbitals in pairs, the two electrons spinning in opposite directions. 



