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In the accompanying Report of the Committee on Luminescence we have made no attempt to cover the whole subject nor to arrange the material presented in such a way as to bring out its bearing upon any one general theory. The large mass of experimental material and the lack of agreement regarding the underlying theory of the subject make it doubtful whether any other method of treatment would be either desirable or practicable.
There can be little doubt that the most important problem at the present time in the field of luminescence is that of developing some satisfactory theory which may serve as an aid in correlating the observed phenomena and as a guide in planning new investigations. The choice of the topics discussed in the different chapters of this report has been largely determined by our feeling that the subjects selected for consideration are of especial importance because of their bearing upon this problem. For lack of time, however, a number of subjects which we should have been glad to include have been omitted from the report. Broadly speaking, each chapter forms a unit by itself.
As defined by Wiedemann the term luminescence includes all cases of radiation except those due to temperature alone. Generally, however, the term is used in a more restricted sense, and common usage is, unfortunately, not always consistent. All writers agree in including fluorescence and phosphorescence, whether excited by light or by other agencies, under the general head of luminescence. There is also no difference in usage in the case of tribo-luminescence and thermoluminescence. But in connection with the emission of light by the arc, by Geissler tubes, and by flames, the use of the term luminescence, except in a non-technical sense, is less common. It is possible that an essential difference does exist between these different cases of emission. But the difference is difficult to define; and it is questionable whether it is not more important to accentuate the fundamental difference between temperature radiation and all other types of light emission than to distinguish between different kinds of non-temperature radiation. We feel that it is preferable to adhere to Wiedemann's definition; accordingly we have used the term luminescence in that sense throughout this report. The topics chosen for discussion, however, lie almost without exception in the restricted field to which the term luminescence is universally applied.
The Bibliography of Luminescence, Chapter VII, has been prepared by Dr. J. A. Becker and is the result of a serious attempt to include references to all books and articles on luminescence that have appeared
between the years 1906 and 1922. Articles previous to 1906 are not included because of the very thorough discussion of the earlier work in Kayser's Handbuch der Spektroskopie. Papers on General Spectroscopy have not been included in the Bibliography except in cases where they appear to have a direct bearing upon the theory of luminescent radiation. Because of the obvious connection between luminescence and absorption spectra, however, the references to work on absorption are quite numerous. We have attempted to make the references to work on flame spectra as complete as possible because of the striking resemblance of these spectra to luminescence spectra excited in other ways. Especial attention has also been paid to work on absorption or emission spectra in which constant interval series occur.
References in the text to books and articles listed in the Bibliography are by number; in the case of books the number is preceded by the letter B.
Professor of Physics, Cornell University
EDWARD L. NICHOLS
Professor of Physics, Emeritus, Cornell University
C. D. CHILD
Professor of Physics, Colgate University
Perrin's Photochemical Theory.
*This committee of the Division of Physical Sciences of the National Research
THEORIES OF LUMINESCENCE
By ERNEST MERRITT
The first attempt to develop a theory of luminescence was made by E. Wiedemann' in 1889 and the suggestions then made were discussed in greater detail in a later paper by Wiedemann and Schmidt2 which appeared in 1895.
According to the Wiedemann theory luminescence is an accompaniment of some reversible molecular change. It is assumed that the active substance is changed by the action of the exciting agency from the stable condition A into the unstable condition B. If the return of the substance to the condition A is accompanied by the emission of light we have phosphorescence. Fluorescence may be due either to vibrations set up during the change from A to B, or to the fact that the change B to A proceeds, with the emission of light, during excitation as well as during decay. Thermo-luminescence is to be explained as the result of some change that is made possible by rise of temperature, during the progress of which the molecules are thrown into such violent vibration as to emit light. In many instances thermo-luminescence seems to be nothing more than accelerated phosphorescence. In such cases the conditions at the temperature of excitation are apparently not favorable for the rapid emission of the energy stored during excitation. Phosphorescence is therefore very weak and of extremely long duration. This may be true in such marked degree that no phosphorescence can be detected at all, and the condition produced by excitation is practically permanent so long as the physical conditions remain the same. In such cases the reaction A to B can occur at the lower temperature, while the reverse reactions B to A cannot. But if the temperature is raised, it becomes possible for the stored energy to be liberated and thermo-luminescence is observed.
While the fundamental conception of the Wiedemann theory has been retained, it has been customary in recent years to make the hypothesis more definite by assuming that the change referred to as the reaction A to B consists in the expulsion of an electron from a molecule of the active substance, while the recombination of the ions thus formed constitutes the reverse reaction B to A. This is the form of the theory that has been advocated by Lenard and others. Chemists, on the other hand, are inclined to look upon the change assumed E. Wiedemann, Annalen der Physik, 37, p. 177, 1889.
E. Wiedemann and C. C. Schmidt, Annalen der Physik, 56, p. 177, 1895. This paper is as important in the history of the theory of luminescence as was the work of Stokes or of Becquerel in the experimental study of the subject.