RECENT XD\--ISCES I S OUR KNOUrI,EDGE OF COLD LIGHT' BE' HERBERT E . IVES
S o more fascinating field of research exists than that of light production by means other than high temperature. Both from the standpoint of scientific theory. and from the standpoint of practical light production, the opportunities for increasing and applying our knowledge are great. On the other hand, there is scarcely any branch of science in which adequate theories are so few or so unsatisfactory. The theory of light production by luminescent processes is thus in striking contrast to the Wien-Planck theory of temperature radiation. The general characteristics of luminescence are, first, the low temperatures a t which the light is produced, and second, the discontinuous spectra of such light. Conforming to these characteristics are the luminous phenomena produced b y a variety of causes, in different media, and under various conditions. The usual classification is into fluorescence and phosphorescence, produced by light, cathode rays, X-rays, etc. ; electro-luminescence, produced by the passage of a n electric current; tribo-luminescence, brought about by friction ; thermo-luminescence, caused b y gentle heating; and chemi-luminescence, accompanying chemical reactions, and including the interesting cases of light production by living organisms. A very large amount of experimental work has accumulated along these lines, and no more than a brief mention of the more striking researches, with their theoretical bearing, can be here attempted. FLuoiescence.-By fluorescence is commonly understood the production of light possessing the characteristics of luminI paper read before the Eighth International Congress of Applied Chemistry in Ne\\ X-ork. September. 1 9 1 2 .
escence while under the influence of the exciting force, which latter may be light, cathode rays, or other form of radiant energy Besides the general characteristics of luminescence above noted, fluorescent light has been thought to conform to Stokes’ law, which states that the emitted light is always of greater n-ave length than the exciting. Aknong the most important recent experimental work on fluorescence must be ranked that of U‘ood, on sodium and other metallic xrapors. By the use of improved technique, and of higher spectroscopic resolving power, the fluorescent spectrum of sodium vapor was found, when white light is employed as excitation, to be of a delicate banded structure, When, however, monochromatic excitation is used, there results not a banded b u t a line fluorescent spectrum. -4s the n-ave length of the exciting light is changed, different sets of line spectra appear, each forming a regular series. I n these series, Stokes’ law is not observed, for by exciting any one line of a series by resonance all the other lines respond, indicating that the various monochromatic radiations are given out b y a common system. Much information as to the nature of the radiating systems will undoubtedly follow from this work and from t h a t now being carried on in continuation of it. .Among the latter researches may be noted the remarkable experiment of changing the line resonance spectrum of iodine vapor into a band spectrum (resembling that excited by white light) by means of the presence of helium gas. In a somewhat different field are the interesting fluorescent spectra found by Goldstein in the aromatic compounds, which exhibit peculiar changes in intensity and character with variations in the excitation and physical conditions. Sichols and hlerritt in an extensive series of studies have determined the distribution of intensity in the spectra of \-arious fluorescent substances, and the effects of temperature thereon. Their latest work, on the distribution of intensity among the narrow bands in the spectra of the uranyl
28
Herbert
E.PLt?a
compounds, shon-s that the envelope of these bands has a shape similar to t h a t of the intensity distribution in the broad. single emission bands of many other fluorescent substances, such as the alkaline earth sulphides. Taken in conjunction with the fact t h a t these bands are much sharper a t low temperatures (Becquerel) and broaden a t high temperatures, there is here evidence for the complex character of the rather common diffuse emission bands given by fluorescent and phosphorescent substances. The chief theories of fluorescence consider it in relation t o phosphorescence and will be treated here under that head, Phosfihovescence. -By phosphorescence is commonly understood a luminescence which persists after the removal of the excitation. Frequently i t is considered as differing from fluorescence in this respect alone and the one is held to graduate insensibly into the other. Certain substances which, on ordinary examination, appear to be merely fluorescent are found on examination with the phosphoroscope to exhibit a phosphorescence of very short duration. Often the duration of this phosphorescence may be increased by lowering the temperature. Attention must here be directed chiefly to two investigators and their theories, and to the work which has been done by them or inspired by them-Kowalski and Lenard The theory advanced by Kowalski calls for two systems of electron groupings, called electronogens and luminophors. The first expel electrons under the influence of light; the second have an internal energy closely approaching the critical value a t which explosive action takes place in accordance with the ideas of J. J. Thomson. Under the influence of light the electronogens expel electrons which carry additional energy to the luminophors, destroy their equilibrium and so cause disintegration and emission of light. Recent experimental work by Konalski on organic compounds a t lon- temperatures has resulted in the separation of two processes, one of which decays with great rapidity on the
removal of the excitation, the other much more slonly. The two processes have difierent spectra. These results are apparently more in accord with the theory of Lenard, n-hich follon-s . Lenard has directed his study to the alkaline earth sulphides, of which Balmain’s paint (calcium sulphide 1 and sidot blende (zinc sulphide) are examples. Through an exhaustive investigation in conjunction with Klatt, he has definitely determined the composition of these sulphides The constitution of a “phosphor” is three-fold : first, an active metal, present in very small quantity; second, an alkaline earth sulphide; third, a flux. Having reduced the preparation of the phosphors to a science, Lenard has conducted noteworthy investigations on the properties and behavior of these substances under different excitation and conditions. Each phosphorescent spectral band has been found to have three phases -a lower momentary phase. a t low temperatures: a permanent phase; and an upper temporary phase a t high temperatures. The characteristics of the permanent phase are that, first, i t gives lasting phosphorescence, and second, it is caused chiefly through excitation b y light a t several isolated spectral regions. The temporary phases are excited by a large range of short wave-length radiation, and are of excessively short duration of phosphorescence. The various bands have their permanent phases a t different temperatures. To explain these phenomena, Lenard has formulated a very striking theory. He considers the action of light to be the photo-electric action, that is, the tearing off of electrons from the active metal by the resonance action of light. These electrons are then captured by the surrounding sulphur atoms. Some are released a t once, others are stored and only gradually returned. The (photo-electric) electrons on their return. vibrating with ever-decreasing amplitude and period, set in motion, by resonance.. at a certain period in their decreasinq sn-ing, another set of electrons in the metal atoms, the latter called “emission electrons.” It is the vibrations of the emis-
30
Herbert E . lves
sion electrons that cause the visible light, of greater waye length than the exciting (Stokes‘ law) The extinguishing action of infra-red radiation is attributed to the fact t h a t the free period of the captured metal electrons is such,as to respond to the longer wave excitati6n The complex molecule is set into vibration and gives up its metal electrons sooner than it otherwise would. Some deductions from this theory have met with strong confirmation. I t has been found, for instance, t h a t the relative wave lengths of the permanent excitation regions for the same active metal may be derived from the dielectric constant of the alkaline earth sulphide. The theory calls as well for a number of bands not to exceed the number of valence electrons, that is, four, and although Pauli has recently found both ultra-violet and infra-red phosphorescent bands this number is not exceeded. .An interesting contribution to this theory has recently been made by Pohl. Lindemann had discovered that the wave length a t which metals respond to the selective photoelectric effect may be calculated from the atomic volume and valence of the metal, on the assumption of a planetary system obeying Kepler’s laws. Pohl calls attention to the fact that the wave lengths which excite the permanent phase of phosphorescence are in the ratio to each other given by Lindemann’s relationship, in which the successive valences 4, 3 and 2 , are substituted, thus supporting Lenard’s view t h a t we ha1.e here a case of the selective photo-electric effect. Electro-luminescence.-The production of light by the passage of an electric current through a rarefied gas belongs in the category of luminescent phenomena, although the chief study has been from the electrical rather than the light standpoint. The past few years have, however, marked the commercial development of electro-luminescent devices, prominent among which are the Moore tubes, containing either carbon dioxide (white light) or nitrogen (yellow light). Lately, tubes of neon hax-e been employed, noteworthy because of the
Recent ;1daarices in Knowledge o j Cold L i g h t low cathode drop and the large portion of the emitted energy which lies in the A-isible spectrum. Chemi-lumi tiescence --That light is an occasional accompaniment of chemical action, even a t low temperatures, has long been known Trautz, in a remarkably exhaustive study. has recorded some hundreds of cases of chemi-luminescence in its various forms of crystallo, tribo, and purely combination luminescence. N o generalizations are possible a t present beyond the observation that velocity of reaction and intensity of luminescence are approximately proportional. Some of the more intense of these reactions, such as that of pyrogallol and hydrogen peroxide, have been the subject of special investigation, b u t in all cases the amount of light produced was very small and difficult of study. Organic-luminescewu--hlore and more a t terition is being devoted to the study of organic luminescence, whether of bacteria, marine organisms, or the luminous worms and flying insects. In these nature has solved the problem of light production in a manner different from that so far achieved b y man. While usually the total light emitted is small, the intrinsic brilliancy is not so small as to make a similar light useless t o man could he copy its mode of production. Perhaps the most significant work done of late has been on the fire-fly. On the physical side, work by Ives and Coblentz has shown the spectrum of the emitted light to consist of a narrow band in the visible region, unaccompanied b y any emission in all the ultra-violet and infra-red which could be studied b y photography or phosphor photography, and with strong evidence t h a t such invisible radiation cannot exist Nor does the radiation possess any of the characteristics of true phosphorescence, such as dependence on light for excitation, or susceptibility to extinction b y infra-red. Interesting work b y Kastle and McDermot on the chemical side has shown t h a t the light-giving power of the insects is affected by many chemical reagents in a marked manner They find the necessary conditions for the production of light
to be the presence of oxygen arid water. Perhaps the most important result of their work has been the discovery t h a t the dry powdered abdominal material of the insects will give out light when moistened, for as much as two years after the insect is dead, proving the emission of light t o be a chemical phenomenon not dependent on life.
