12 



ELECTROMAGNETIC COUPLING 



An electronic excitation can also be looked upon as an oscilla- 

 tion. Two molecules, the electrons of which are capable of a 

 similar excitation, can thus act as coupled oscillators. In this case 

 it is not necessary to have a material connection between the two, 

 for the electromagnetic field couples them, provided their mutual 

 distance is not too great (small compared to the corresponding 

 wavelength ) . 



The transmission of excitation energies between molecules 

 through electromagnetic coupling is not a mere matter of specula- 

 tion. It is one of the basic theories by which classic phenomena, 

 such as the "concentration depolarization" of fluorescences are ex- 

 plained. It has been found, as a general rule, that if a dilute (e.g., 

 10"* M) solution of a fluorescent substance is cooled to low tem- 

 peratures to form a solid glass in which the single molecules have 

 a fixed orientation, then the fluorescent light given off by this 

 system will be polarized if polarized light was used for its excita- 

 tion. The molecules being fixed in space, they will not change the 

 plane of polarization in the interval between the absorption and 

 the emission of the photon. If, however, this experiment is re- 

 peated with a high concentration of the fluorescent substance (e.g., 

 10"^ Af), then the fluorescent light emitted will not be polarized. 

 The generally accepted explanation is this: owing to the high con- 

 centration, the molecules are now close to one another; being 

 coupled by the electromagnetic field, the E* is passed from one to 

 the other before it is shot out again as a photon. The different 

 molecules visited by JB* have a random orientation, thus the fixed 

 relation of the plane of the primary exciting light to that of the 

 fluorescent light will no longer be maintained and the polariza- 

 tion will be lost. It will be lost if the statistical distance between 

 molecules is smaller than the distance at which such resonance 

 transfer of excitation is possible. Figure 3 shows the relation of 

 concentration to the statistical distance between molecules in any 



