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Bibliography
- P. Pringsheim and M. Vogel, Luminescence of Liquids and Solids and Its Practical Applications, p. 202, X, Interscience Publishers, Inc., New York, 1943.
P. Pringsheim and M. Vogel, Luminescence of Liquids and Solids and Its Practical Applications, New York, 1943.
- Yack de Ment, Fluorochemistry, p. 796, XVIII, Chemical Publishing Co., Inc., Brooklyn, 1945.
J. de Ment, Fluorochemistry, Brooklyn, 1945.
The general character of the books under review is determined by the fact that, over the last 10–15 years, the circle of readers interested in the phenomena of luminescence has expanded to an unusual degree. Whereas until comparatively recently the phenomena of luminescence had almost no practical applications and attracted the attention only of a rather limited circle of specialists, in recent years—literally before our eyes—the picture has changed radically. These phenomena have acquired a number of applications of first-rate importance: fluorescent light sources have become widespread; methods of luminescence analysis, extensively disseminated, have come to serve the needs of industry; screens for all kinds of cathode-ray devices, etc., and, in connection with this, among very broad circles of engineers and chemists, geologists and mineralogists, physicians and biologists, there has arisen a need for literature that would give them the opportunity to broaden and systematize their knowledge of the whole distinctive field of phenomena with which they come into contact in their practice. Demand of this kind inevitably had to bring forth a corresponding supply, and indeed we may note that all the monographs on luminescence that have appeared in recent years on the world book market are oriented not so much toward the specialist physicist working in this field as toward the wide circle of representatives of related specialties who feel the need for a general acquaintance with the phenomena of luminescence and especially with their practical applications.
Especially characteristic in this respect is the book by Pringsheim and Vogel. Pringsheim’s name is widely known to everyone working in this field, both from his own numerous and important studies and from the monographs and surveys he has written (Fluorescence and Phosphorescence, 3rd ed., 1928, and the survey in the Handbuch der Physik). Pringsheim’s new book, written jointly with Vogel, is by no means a reissue or revision of the earlier monograph; on the contrary, it differs from it in a fundamental way. Whereas in the earlier monograph attention was focused on a detailed presentation of the results of experimental work and on elucidating the mechanism of various luminescence phenomena, the book under review is constructed according to a completely different plan. All theoretical questions are presented concisely in the first two chapters, which cover only 32 pages in all. These are followed by a considerably larger chapter (30 pages) devoted to experimental technique, and by a chapter of approximately the same size containing a brief description of the principal luminescent materials and their properties. This exhausts the first part of the book, “Physics of Luminescence.” The second,
part—“Applications of Luminescence”—contains two chapters: “Luminescence Analysis” and “Luminescence as a Source of Light” (luminescent paints, screens, and luminescent lamps).
The greatest interest in the book by Pringsheim and Vogel is, of course, the first, physical part. The authors did not set themselves the hopeless task, within the given scope, of covering all, or even the most important, experimental results and their physical interpretation. They strove above all to formulate clearly the basic concepts with which one has to deal in studying the phenomena of luminescence, and with this limited but very important task they have coped completely. After a very brief historical introduction (ch. I), a perfectly correct and rigorous distinction is drawn between the phenomena of thermal radiation and luminescence; the points at which not only poorly prepared readers, but many specialists as well, become confused are pointed out, and a reasonable classification of these phenomena is given, essentially adjoining the classification given by S. I. Vavilov. A weak point here is the insufficient clarity in the exposition of the question of the connection between the “monomolecular” or “recombination” character of the emission, on the one hand, and the laws of decay of one or another kind of emission, on the other. The reader may get the impression that in the first case the law of decay is always exponential, while in the second it always represents a second-order hyperbola. Both, as is known, are incorrect. The kinetics of the process of phosphorescence is, in general, not touched upon at all in this book; this, of course, is a serious omission, for this question not only has very great practical significance, but is also most closely connected with the mechanism of the phenomenon. On the other hand, the authors’ indication of the fact that the difference between two types of decay curves lies not so much in the form of these curves as in their dependence on temperature and on the initial value of the intensity of the emission is very useful.
Further in this chapter the question of the excitation of luminescence by various factors, and of the yield and intensity of luminescence, is considered. The factual material given here is very limited, but an undoubted and great merit, from the point of view of the general aim of the book, is the very precise and clear distinction between the concepts of light yield, energy yield, and quantum yield of luminescence—concepts in which nonspecialists often become confused. At the end of this chapter, in only two pages but with great clarity, the relation between luminescence and the chemical reactions accompanying it and other processes is formulated.
The chapter on experimental technique, of course, cannot equip the reader and fully prepare him for work in this field, but nevertheless it contains a fairly clear and precise survey of modern means of investigation and will undoubtedly be useful at a first acquaintance with the subject. Modern photoelectric methods of measurement are touched upon in it, although rather superficially. Thus, for example, photomultipliers are mentioned only briefly, in a footnote, while modern constant-current amplifier circuits with the use of electrometer tubes and methods of compensating anode current, which ensure increased stability (DuBridge circuits, etc.), are not mentioned at all.
Of perhaps the greatest interest is ch. IV, devoted to a survey of luminescent materials. Here the following very logical classification of these materials is established: 1) organic compounds, 2) pure inorganic compounds, 3) synthetic inorganic phosphors, 4) minerals and glasses. In a concise and at the same time very rich form, a survey is given of the principal properties of these materials. Particularly successful is a small section devoted to inorganic compounds. This section will be read with great benefit not only by the begin—
ing reader, but also every specialist. The section on crystallophosphors was written, it is clear, “by others’ hands.” A significant shortcoming of it is the complete absence of information on present-day views concerning the mechanism of phosphor luminescence, connected with the concepts of the band theory of semiconductors, as well as the already noted absence of data on decay curves and on the duration of phosphorescence.
The second part of the book gives a sufficiently complete and modern general survey of the principal applications of luminescence. The reader for whom the book is intended will read this part with great profit. Here the question of luminescence analysis, associated with the fluorescence of organic substances, is also closer to the authors and is presented with greater depth and originality than the question of the lighting-engineering applications of crystallophosphors.
On the whole it must be acknowledged that the book by Pringsheim and Vogel represents a very useful contribution to the literature on the subject, and that it can safely be recommended both for a first general acquaintance with it and for reading by specialists, who will find in it many fresh and original thoughts scattered between the lines.
Both in its very substantial size and in its general conception, de Menth’s book differs quite essentially from Pringsheim’s book. The tasks that the author set himself are clearly formulated by him in the preface. The author indicates that he was deeply impressed by Poincaré’s words to the effect that the collection of facts is no more science than a heap of bricks is a building. Inspired by these words, the author decided to systematize the material and to give a connected exposition of this branch of our knowledge, which, in his opinion, has for far too long represented rather “a monstrous heap of facts interspersed with references and assumptions” than a science. Meanwhile, in the author’s opinion, the theory of these phenomena and their experimental study have already advanced so far that the time has come to single them out into an independent new scientific discipline, for which de Menth also proposes a new name: “fluorochemistry.”
One may, of course, disagree with the author that the totality of luminescence phenomena should be treated as an independent discipline and not as one of the chapters of physical optics. One may also regard as extremely unfortunate the name proposed by de Menth for this new discipline, if only because, in essence, the study of luminescence phenomena relates more to the domain of physics than of chemistry. But in any case it is beyond doubt that a systematic and coherent survey of the enormous accumulated experimental and theoretical material would be exceedingly useful and timely—provided, of course, that the quality of the execution of this plan corresponded at least in some measure to its grandeur.
Unfortunately, however, it cannot be said that de Menth’s book satisfies this condition. The task the author set himself clearly exceeds his powers, and the result is a book which, in fact, more closely resembles a pile of building material—and often a wholly unfit one—than the construction of a new scientific discipline.
Especially weak is the first, most important part of the book, entitled “Fluorochemistry in Theory” and divided into two chapters: “The Physical Aspect of Luminescence” and “The Physico-Chemical Aspect of Luminescence.” It is hard to imagine anything more unsystematic, ignorant, and at the same time pretentious than these 140 pages of de Menth’s book!
In order not to be unfounded, let us dwell on a few examples. We emphasize that these are not isolated accidental errors that have slipped into an exposition correct on the whole, but precisely examples illustrating the whole level of the book—examples whose number could be increased at will.
Bibliography
Let us begin with the very definition of the subject of the newly emerging scientific discipline. The author is completely unclear about the fundamental and profound difference between the phenomena of thermal and luminescent radiation. He mixes both of these phenomena into one heap, adds to them the phenomena of cathodoluminescence, and reduces the differences among all these kinds of luminescence to differences in the temperature at which they are observed. The result is something monstrously illiterate! “It is necessary to distinguish,” the author writes, “two kinds of luminescence. One, connected with temperatures roughly exceeding 1000°, is cathodoluminescence. The mechanism taking place in the filament of an incandescent lamp illustrates this type of emission of light, which is a direct result of temperature (‘thermal luminescence’). The other, fluorescence, is indeed a cold process, usually occurring at temperatures below 1000° (‘nonthermal luminescence’). Both types are closely connected with one another…”
After this definition of the subject of the investigation follows the formulation of three fundamental laws on which, as on three whales, the edifice erected by the author is supposed to rest. The first of these laws is the extraordinarily pompously presented “law of absorption of de Ment,” or “first law of fluorescence.” “In 1942,” we read, “de Ment formulated a fundamental principle governing all processes of a luminescent nature. Based solely on theoretical considerations, the first law of fluorescence asserts that before the emission of light by a luminescing system can occur, absorption must take place in it” (emphasis by the author.—L. T.). Thus, before 1942, and despite a number of theoretical reflections, de Ment did not know that in the phenomena of photoluminescence the emission of light is preceded by absorption, and only de Ment established this new and striking fact. De Ment emphasizes the parallelism between his law and the well-known photochemical law of Grotthuss–Draper. This observation is correct, but one should not lose sight of the fact that Grotthuss expressed his law in 1818, and Draper independently arrived at the same point of view in 1839, i.e., long before the concept of energy had crystallized and the law of its conservation had been found. At that time the Grotthuss–Draper law did indeed play an important role in the development of ideas about the essence of photochemical reactions. But in the middle of the twentieth century statements like the “law of de Ment” constitute triviality, having no purpose whatsoever, while the author’s pretensions to a great discovery are simply ridiculous.
Mathematically the author formulates his “law” in the form of the formula:
\[ \frac{dc}{dt} = \]
\[ = K \cdot I_p \left(1 - e^{-\mu' c d}\right), \]
where \(\frac{dc}{dt}\) is the “rate of release of luminescent light.” What this means and what sense the written formula has is impossible to understand. In my opinion, it means nothing, and the formula is one-hundred-percent nonsense. In the preface the author expresses gratitude to Dr. Stamer Shabk for help in the mathematical treatment of certain fundamental relations. Judging by the formula cited, it is hardly possible to regard this gratitude as deserved.
As the second fundamental law of fluorescence de Ment regards the well-known Stokes law. However, he manages to present to readers even this widely known law in a completely distorted form. According to de Ment, Stokes law asserts that “the energy emitted in luminescence always exceeds the energy absorbed during its excitation.” (Emphasis by the author.—L. T.) Such an energetic interpretation of Stokes law, of course, completely distorts the historical perspective. Not to mention the fact that the very concept of energy was only being born at the time when Stokes law was formulated (1852), the idea of the connection
between the frequency of radiation and its energy there is an idea that is specifically quantum, utterly foreign to classical, purely wave optics. Stokes formulated his rule as a purely spectral rule. As such, later researchers, in particular Lommel, also tried to explain it. And only Einstein, on the basis of photon conceptions of the nature of light, linked the question of the wavelength of the absorbed and emitted light with the magnitude of the energy of the corresponding quanta.
The subsequent exposition is developed at the same level; its chaotic and unsystematic character is combined with the grossest errors. So as not to weary the reader, we shall confine ourselves to a single, entirely anecdotal example. On p. 105 the author gives the well-known Perrin–Vavilov formula for rotational depolarization of luminescence, i.e., for depolarization caused by Brownian rotation of a molecule during its residence in the excited state. This formula contains the factor
\[ \frac{RT}{V\eta}, \]
where \(R\) is the gas constant, \(T\) is the absolute temperature, \(V\) is the molecular volume of the fluorescent substance, and \(\eta\) is the viscosity of the solvent. De Ment, however, with splendid self-assurance, declares that the quantity \(R\) in this formula is... Rydberg’s constant (?!).
We believe that what has been said is sufficient to judge de Ment’s book. It should not merely not be recommended to a chemist, engineer, or physician who wishes to become acquainted with the phenomena of luminescence; on the contrary, they must be warned in every way against using it, for in a nonspecialist reader it can engender only utterly chaotic, confused, and profoundly incorrect notions of these phenomena.
The next six hundred-odd pages of de Ment’s book constitute a survey of an enormous number of experimental works. The value of this survey is considerably diminished by the fact that it is completely unsystematic and devoid of a critical approach to the material presented. The impression is created that diligent bibliographers copied onto cards a vast number of summaries of works on luminescence, and then these cards were put into the book without any understanding of the essence of the matter, the mutual relation of the works, and their relative value. Of course, even in this form the survey cannot be regarded as absolutely useless. For example, if it is impossible to use the original Colour Index, the reader will be grateful to de Ment for the extensive (35-page) excerpt from this fundamental handbook. Likewise, it may happen that, on looking through de Ment’s book, a specialist will find mention of one or another work that had escaped his attention, or will come upon a bibliographic reference he lacked. With this, however, the usefulness of the book is exhausted. The book cannot be recommended even as a somewhat systematic and authoritative source of information.
L. A. Tumerman