CHAPTER XV. 



THE PHENOMENA OF PHOSPHORESCENCE CONSIDERED FROM 

 THE STANDPOINT OF THE DISSOCIATION THEORY. 



It is our purpose in this chapter (i) to derive the law of decay of phospho- 

 rescence which the dissociation theory would lead us to expect under such 

 ideally simple conditions as seem to exist in the case of gases; and (2) to 

 consider in turn the various modifying factors which may have an influence 

 on the phenomena of phosphorescence in solids, and to determine so far 

 as possible the nature of this influence. Comparison with experimental 

 results will then make it possible to form an opinion of the relative impor- 

 tance of the various factors considered. 



It will facilitate the discussion if we consider first the requirements which 

 a satisfactory theory must meet. The most important experimental results 

 in the case of photo-luminescence are briefly mentioned below : 



SUMMARY OF EXPERIMENTAL LAWS. 



1. Stokes's Law. 



2. If we isolate a single band of the luminescence spectrum it is found 



that the distribution of intensity throughout the band is inde- 

 pendent of the intensity and wave-length of the exciting light. 



3. The light emitted by an isotropic substance during luminescence is 



unpolarized, no matter what may be the condition, as regards 

 polarization, of the exciting light. 



4. During the decay of phosphorescence each band of the luminescence 



spectrum behaves as a unit; i. c, the wave-length of maximum 

 intensity and the relative distribution of intensity throughout 

 the band remain unchanged. 



These general laws were discussed in the preceding chapter and it was 

 pointed out that they are directly deducible from the dissociation theory. 



To these four general laws must be added the following experimental 

 facts connected with the decay of phosphorescence : 



5. Form of Decay Curve. The curve obtained by plotting the values 



of I~- as ordinates and the corresponding values of /as abscissas 

 is a straight line for small values of t; it changes to a curve con- 

 cave toward the axis of / as / increases; but for still larger values 

 of / the relation between I~- and / is again linear, and remains 

 so until / becomes too small to measure. The form of the decay 

 curve is dependent on the intensity and duration of excitation, 

 the slant being altered in each of the straight parts by a change 

 in either of these two factors in the excitation. 



6. Hysteresis. The behavior of a phosphorescent substance with a 



given excitation depends upon its previous history. Some semi- 

 permanent change is produced by excitation which persists for 

 several hours, or even for several days, after visible phosphores- 

 cence has ceased. 



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