82 STUDIES IN LUMINESCENCE. 



linear relation that holds for the green band alone. Yet the deviation from 

 a linear relation in both these eases is well within the errors of observation. 

 Again, if the infra-red rays increase the brightness of the violet band after 

 excitation has ceased it would seem reasonable to expect a similar effect 

 during excitation. Yet the effect during excitation (Fig. 67) is nearly the 

 same for both bands. 



We were first led to expect increased brightness in the violet during 

 exposure to the infra-red, and to undertake experiments in the hope of 

 detecting such an effect, as the result of an entirely different line of reason- 

 ing. Looking upon phosphorescence as due to the recombination of ions 

 dissociated by the action of the exciting light, we may explain the fact 

 that the phosphorescence light is of greater wave-length than the exciting 

 light (Stokes's law) briefly as follows : Dissociation results from the violent 

 resonant vibration of a neutral molecule of the active substance under the 

 influence of the exciting waves. The wave-length of maximum resonance 

 and therefore maximum excitation is determined by the natural period of 

 vibration of the active molecule, which is influenced to some extent, but 

 not greatly, by the surrounding solvent. The charged ions resulting from 

 excitation will, however, be attracted by the neutral molecules of the sol- 

 vent and will form the nuclei of heavy aggregations of molecules; and 

 recombinations of the ions will therefore occur under conditions which 

 make the resulting vibrations longer, on the whole, than the period of the 

 active molecules before dissociation; hence the well-known displacement 

 of the luminescence spectrum with reference to the absorption spectrum. 



Now the effect of the infra-red rays may be to so shake up the molecules 

 of the solvent as to prevent the loading down of the ions by the attraction 

 of neutral molecules, or to destroy such heavy aggregations if already 

 formed. Under the influence of the infra-red, therefore, the light emitted 

 will be due largely to the vibrations that occur during the recombination 

 of unloaded ions and will be of the same wave-length as that which the 

 active substance absorbs. If the screen is exposed to infra-red rays after 

 excitation we should expect a decrease in the intensity of phosphorescence 

 throughout the phosphorescence band due to the breaking down of the 

 groups of molecules referred to above. But owing to the resulting increase 

 in the number of unloaded ions we should also expect the emission of light 

 whose wave-length is that of the resonant absorption band of the substance. 

 Now the absorption band always lies on the ultra side of the luminescence 

 band, and usually the two bands overlap. (That this is the case with Sidot 

 blende is shown by the fact that this is one of the substances for which 

 vStokes's law, in its strict form, is violated.) Exposure to infra-red should 

 therefore produce increased intensity near and beyond the violet edge of the 

 phosphorescence band; which is exactly what we have observed. 



In the region where the absorption and emission spectra overlap, the 

 effect will be more complicated. While the light in this region due to the 

 ordinary luminescence band will diminish, there will be at the same time 

 a temporarily increased emission due to the recombination of the unloaded 

 ions that are shaken loose by the infra-red vibrations. A bright flash 

 immediately after the exposure to infra-red, followed by decay more rapid 

 than the normal, is therefore to be expected in the intermediate region 

 corresponding to Figs. 73 and 74. 



