# Popular Science Monthly/Volume 83/September 1913/The Absorption and Emission Centers of Light and Heat

 THE ABSORPTION AND EMISSION CENTERS OF LIGHT AND HEAT.
By Dr. W. W. STRONG

UNIVERSITY OF PITTSBURGH

THE mechanical motions of nature are transmitted by solids and fluids from sources that consist of more or less well known mechanical systems. Waves on a pond may be due to a boat moving over the surface of the water. Sound waves in air may be due to the vibrations of a tuning fork. Wireless telegraph waves may be due to high frequency electromotive force and current waves in electrical circuits. In general the source of the above type of wave motion is a kind of mechanism that can be made in the laboratory or in the shop—a mechanism that is man-made and whose operation is quite obvious to us.

The phenomena of light and radiant heat introduce to us a type of wave motion that is altogether different. Not only may the medium that transmits this wave motion possess entirely different properties from that of matter, but the mechanisms that take part in the emission and the absorption of the wave motion are altogether different from any that we have been able to make in our laboratories. No one has succeeded in producing radiant heat, much less visible light and ultraviolet radiations by means of electromagnetic oscillator, although such a feat may be possible.

Inasmuch as matter is the source of all heat and light radiations, the mechanism responsible for the emission and absorption of these radiations must be intimately related to the nature and constitution of matter itself and therefore theories of emission and absorption systems depend to a large extent upon our theories of the nature of atoms and molecules. It must be remembered, however, that the nature and constitution of atoms and molecules that explain chemical and many other phenomena need not necessarily be at all related to the systems taking part in heat and light radiations.

In the past many different hypotheses have been advanced to extend the atomic and molecular conceptions of Dalton, Clausius, Maxwell and others. As long as the elastic solid theory of the ether prevailed, it was frequently assumed that the vibrating systems emitting light and radiant heat were of a mechanical nature due to the development of stresses and strains. The electromagnetic theory of Maxwell, while not even suggesting the nature of the mechanism, metamorphosed our views of radiant energy and indicated the whole phenomenon to be an electromagnetic one.

Black Body Radiations and Electron Atmospheres

Since the advent of the theories of liquid and gaseous ionization, many attempts have been made to construct a system composed of ions and electrons of various kinds that would be capable of explaining the phenomena of optics and radiant heat. In the case of black body or pure temperature radiations, the theory has been quite successful and seems to correctly describe the actual conditions. Solids or liquids are known to contain largo numbers of electrons and when these bodies conduct metallically there is good reason to believe that the electrons move about in these bodies like gaseous molecules in a gas, the law of the equipartition of energy applying to an electron "gas" in a metal in the same way as it does to gases outside the metal. The emission of light and heat under these conditions is presumably due to the production of electromagnetic waves when the electrons are greatly accelerated or retarded in their motion. Laws of radiation like those of Wien and Planck can be derived from the conditions that would be expected to hold in an electron atmosphere. In this type of radiation the distribution of energy throughout the various wave lengths is practically independent of the kind of matter, but depends only upon the temperature and the nature of the electron atmosphere. Thus the radiation constants are universal constants depending upon one kind of radiating and absorbing system, the electron.

Many sources of light and radiant heat emit radiations whose energy distribution over the various wave lengths is very different from that of a black body radiation. These radiations are selective and depend upon the nature of the body that is emitting or absorbing. Emission spectra illustrating this selective radiation are spark, arc, band and other spectra. Colored objects all show selective absorption. The problem of unraveling the constitution of the centers of selective radiation and absorption is a very difficult one and at present many efforts are being made to correlate the possible constitution of such centers with the ordinary molecular, atomic and ionic theories of matter. During recent years the trend of theory has been largely directed towards the view that emission and absorption spectra originate in systems that have a more or less momentary existence, owing to the fact that such optical systems are essentially dynamic in nature. It is very natural, therefore, that especial efforts should be made to find the existence of these momentary systems during periods of ionization and recombination of atoms, molecules, ions and electrons.

An Ideal of the Illuminating Engineer

The subject of selective emission and absorption is one of prime importance to the illuminating engineer. The rods and cones of the retina are selective absorbers of light. Any illumination should, therefore, be tuned to this selective absorbing mechanism of the eye. Under these conditions the illumination will be most pleasing and there will be a minimum amount of energy used in the emission of the radiation used for illumination. Naturally this kind of radiation will be a "cold" radiation and not a temperature one. It is represented in nature by the light from glow worms and fireflies and in laboratories, approximately, by various kinds of phosphorescent materials, the source of such radiation being at room temperature.

Definition of Emission and Absorption Centers

The problem of finding the constitution of the emission and absorption centers of selective types of spectra such as those of phosphorescent substances, sparks, arcs, flames, etc., is a very difficult one, and at present many efforts are being made to correlate the possible constitution of such centers with the various molecular, atomic, ionic and electronic theories of matter. Emission and absorption centers of light and heat are the smallest particles or entities from which one can obtain any given characteristic emission or absorption spectrum. A further division or change of the centers will result in making it impossible for the given spectrum to be emitted or absorbed although the resultant particles or entities may possess a characteristic spectrum of their own. From the definition it is to be noticed that the centers need not necessarily be matter, i. e., possess mass. "When the centers move with reference to the observer, their spectral lines and bands will show the Doppler effect.

Light centers seem to be very complex in their nature. Professor Rowland used to compare them to a piano and the work of Professor Wood upon resonance and fluorescent spectra indicate that the analogy is quite an appropriate one. Strike a key, i. e., excite a vapor like that of sodium with monochromatic light and a whole set of harmonics will be set into vibration. In the case of sodium vapor, each series of lines or bands seem to be due to vibrations of systems that may be quite independent of each other. Apparently there are a large number of these vibrating systems in the light centers of the fluorescent spectra of sodium. The center itself may correspond to the atom of sodium, though at the present time no definite evidence has been brought forward to prove that the center is even of atomic magnitude.

The Problem of Light and Heat Centers

In the study of light centers, attention must be directed for a moment to the many and serious difficulties connected with the problem of determining the nature and constitution of these particles or entities. The conditions under which they exist are very different from the conditions under which we study the other physical and chemical units of matter. Then again, it seems that light centers have a comparatively enormous absorbing or emitting power, so that only a small part of the matter in a given region is concerned with the light and heat emitted or absorbed in the region. Light beams that are sufficiently intense to study are apparently emitted by a very large number of light centers and for this reason it has been found impossible to isolate individual centers; and even if this were possible it may be that the life of these centers is so short that even the isolation of centers would not permit their being studied. Then again, light emission is usually accompanied by many intricate phenomena such as ionization and chemical reactions and this adds to the complexity of the problem. When we consider our profound ignorance respecting even the nature of chemical forces, the constitution of the molecule and atom, the nature of the electric and magnetic fields and even the nature of light itself, it is not at all remarkable that little definite and certain knowledge has been obtained concerning the nature of light centers.

Some Methods of Approaching the Problem

There are several avenues of approaching the problem of the nature and constitution of light centers that seem to be extremely inviting.

1. At the present time a wonderful field is being opened concerning the dynamics of chemical reactions. As chemical reactions are intimately related to heat and light effects, the discoveries in this field are bound to give a great deal of information concerning light centers.

2. A study of the far infrared promises to break the gap between electromagnetic waves and radiant heat and light centers will probably be found to consist of molecular systems vibrating in a way similar to that of the sources of electromagnetic waves. At the present time we can compare light and heat centers with more or less well-known aggregates of matter and make as many identifications as possible.

3. The separation of complex line and band spectra into series of related lines or bands promises to give us a great deal of information ultimately as to the nature of the vibrating centers, although at present the problem is so complex that no one has been able to devise any mechanism or structure that is adequate to explain the known phenomena. The theory of Ritz has been one of the most successful so far advanced.

4. The Zeeman effect obtained by placing the heat and light centers in a magnetic field is important. This effect indicates that many of the centers of spectral lines consists of negative electrons.

5. The Humphreys-Mohler pressure shift of spectrum lines, the Döppler shift of lines and bands emitted by moving centers as studied by Stark and others, are also very important.

Possible Structure of Light and Heat Centers

One may picture light and heat centers as consisting in part as follows:

1. Neutral "aggregates" of charged particles possessing, in general, translatory and rotatory energy. When undisturbed from without these aggregates would have little if any external electric field. When the equilibrium of such a system is disturbed by collisions or by electromagnetic waves, it may possess temporary fields that will serve as the source of heat and light radiation. This radiation may be due to a rapid oscillatory motion that may be radial, transverse or tangential and would probably be characterized by a definite period. On account of the magnitude of the forces necessary for stable equilibrium, the period of the radiation would probably be small. The spectroscopic models of Thomson, Nagaoka and others are of this type.

2. "Aggregates" may possess charged parts; these may be so far apart from each other that local fields of considerable intensity may exist. If such an aggregate were to rotate, an alternating electric field would result and radiations would be emitted. This radiation, depending on a central acceleration, would vary in period with each impact, so that the various periods emitted would vary about a mean, which would depend on the average rotational energy before impact and the nature of the impact.

3. Freely charged particles torn from neutral "aggregates" will radiate energy when their velocity is changed. The quantity of this radiation will vary with the velocity and the acceleration. The breaking up of the neutral "aggregates" may be called ionization if the resulting parts are charged. Ionization processes may take place within molecules and this is believed to be the condition existing in many kinds of organic compounds when they absorb light or heat.

The relation between ionization and luminosity is not yet clear. Some physicists believe that the two are related to each other and that luminosity becomes perceptible when the intensity of ionization is sufficiently great. It has been stated that a gas may become luminous when one molecule in every ${\displaystyle (10)^{7}}$ is ionized. This would mean that the (expenditure of about ${\displaystyle (10)^{-5}}$ ergs is necessary to excite luminosity.

Ionisation and Light Emission and Absorption

There appears to be considerable evidence supporting the view that some band spectra such as those of bromine and iodine may be due to the dissociation of molecular systems or to a recombination of the dissociated parts. Ladenburg has found that luminous hydrogen gives an anomalous dispersion in the neighborhood of ${\displaystyle {\ce {H_{\alpha}}}}$ while this kind of dispersion is absent in ordinary hydrogen. The phenomena of dispersion indicate that different series of lines in a spectrum may be emitted by very different kinds of vibrating centers, while a particular center may emit only a single line of a series, depending on the manner of its excitation. Faintness in the intensity of lines may be due to the fact that there are very few light centers emitting the given line, or that the vibrations have only a very small amplitude. Koenigsbcrger and Küpfur and others consider that the band spectra of iodine, bromine, nitrogen peroxide ${\displaystyle {\ce {(N2O4)}}}$, sulphur, iodine trichloride, nitrogen, etc., are due to a dissociation or recombination of the respective molecules, atoms or ions. In the case of iodine this change might be represented by the equation

${\displaystyle {\ce {I2\rightleftarrows I{}+I}}}$.

At about 800° C. this reaction is about complete and the fme-banded absorption spectra should therefore disappear. Galitzin, Wilip, Evans and others have shown that the bromine absorption spectrum disappears as dissociation becomes more and more complete.

Canal Rays

Canal rays have their source in positive ions that start in front of the cathode, move towards the cathode and pass through any openings in it with a velocity of about ${\displaystyle (10)^{8}}$ cms. per sec. After passing the cathode the canal ray particles may lose their charge or even become negatively charged. The spectrum lines of hydrogen, nitrogen, mercury, sodium, potassium, etc., emitted by canal rays show the Döppler effect when they are viewed in the direction in which the canal ray particles are moving. Accompanying the shifted lines are lines showing no displacement, "rest" lines due to centers that are comparatively at rest. The "rest" line is usually narrow while the shifted line, due to rapidly moving centers, is rather wide, the violet side of the line often being the sharpest. The width of the line indicates the range of velocity of the canal-ray emitting centers. Making certain assumptions as to the potential gradient through which the centers have passed. Stark has calculated the charge carried by centers emitting the various lines.

Since the "rest" and "shifted" lines are separated by a dark region. Stark concluded that canal-ray centers can only radiate line spectra when their velocity exceeds a certain critical value, this critical velocity increasing as the wave-length decreases. Increasing the purity of the gas increases the relative intensity of the "shifted" lines. Strosser has caused a stream of canal-ray centers to impinge into a current of a foreign gas. The "lines of the foreign gas were found to increase in intensity on leaving the cathode, pass through a maximum and then decrease in intensity. The intensity of the lines of the canalray centers decreased in intensity as the distance from the cathode increased.

Carriers of Spark Spectra

Spark spectra have been photographed on rapidly-moving films by Schuster, Hemsalech, Schenck and others. The length of time the metallic vapor continued to emit line spectra was found to vary from ${\displaystyle 10}$ to ${\displaystyle 45(10)^{-6}}$ secs., depending on the line. The velocity of the centers of Mg ${\displaystyle \lambda }$ 4481 was found to be about ${\displaystyle 2.5(10)^{5}}$ cm. per sec. near the electrodes, dropping to ${\displaystyle 1.7(10)^{5}}$ cm. about a millimeter from the electrodes. Air line centers have an existence of about ${\displaystyle 7(10)^{-7}}$ sec. The emission centers in flames and arcs have been studied by Lenard and others. The results obtained do not agree with those found by Stark working with canal rays.

Negative Electrons as Emission and Absorption Centers

The Zeeman effect produced by the action of a magnetic field upon the emission or absorption light centers shows that for many spectrum lines of gases and vapors the light center consists of a negative electron and the ratio of the charge to the mass of the electron obtained in this way agrees very well with the value obtained by other methods. The more accurate experiments give ${\displaystyle e/m=1.775}$ while direct experiments give 1.772.

The Positive Electron

The positive electron has never been isolated in any experiment with vacuum-tube discharges, radiations from radioactive materials, etc. The Zeeman effect of certain band spectra of chlorides and fluorides of some of the alkaline earth elements studied by Dufour and of the absorption spectra of neodymium and erbium compounds as studied by Becquerel indicate the existence of positive electrons. These Zeeman effects may be explained, however, as being due to induced magnetic fields being set up in the region of the light centers, magnetic fields whose intensities are very different from the field impressed from without.

Absorption Centers of Solutions of the Rare Elements

Many solutions of salts of elements such as uranium, neodymium, erbium, somarium, etc., show a banded absorption spectrum. Many of these bands are very narrow. Jones, Anderson and the writer have found that the absorption centers of many of these salts (e. g., uranous chloride) consist of centers containing the salt and an "atmosphere" of the solvent, the whole center apparently acting as a compound. Thus in the above case it is possible to have "water and alcohol centers" of uranous chloride in a solution of uranous chloride in water and alcohol. Increasing the amount of one solvent appears to increase the relative number of the centers of that solvent without apparently changing their composition. The different solvent centers have different degrees of persistency. The water and alcohol bands of neod}Tnium chloride are of about equal intensity when the salt is dissolved in a solution containing about 3 per cent, water and 97 per cent, alcohol. Changes of temperature change the relative persistency of the light centers.

In the case of some uranyl salts the addition of free acid of the salt causes a shift of the bands. This has been explained by the writer as being due to the fact that the light centers consisted of "aggregates" of salt and acid. Evidences of series of "aggregates" were obtained by spectrophotographs of chemical reactions, spectrograms of the absorption spectra of a solution of a given salt being taken as increasing amounts of some other kind of acid was added to the solution.

Centers of Phosphorescent Spectra

Lenard, Klatt, Urbain and others have studied the phosphorescence of various calcium phosphates of bismuth, manganese, nickel, etc. Lenard and Klatt have proposed the view that these light centers or "dynamids" store electrons, the state of motion of the electrons depending upon the temperature. At high temperatures the electrons possess a much greater freedom of motion than at low temperatures. They visualize the states of motion as being "gaseous," "liquid" and "solid." In the "gaseous" state the electrons can occasion the conduction of electricity between the atoms if the latter exist in the same way as they do in metals. In the "liquid" state the electrons are in a state of motion sensitive to light vibrations and therefore they take part in light absorption. In the "solid" state the electrons take part neither in conduction nor in absorption. At low temperatures the spheres of action of the "dynamids" are considered to extend to greater distances than at high temperatures and the free paths of the electrons are therefore greatly reduced.

To each phosphorescent band Lenard and Klatt assign three phases: An upper momentary or heat phase; a permanennt phase possessing quite definite temperature limits; and a lower momentary or cold phase. These phases succeed each other as the temperature falls. The upper momentary phase results when the dynamids do not store electrons. Whenever electrons are stored these return afterwards to the atom from which they were expelled by the light-wave, thus producing the permanent phase of the phosphorescent band. At low temperatures a few electrons return to the atoms from which they were expelled and these cause the lower momentary phase.

The phenomena of phosphorescence are generally conceded to be due to some kind of electrolytic dissociation or ionization of the dissolved substance in the medium about it. Among the first to hold this view were Wiedeman and Schmidt. The theory explains the law of Stokes and many of the other phenomena of phosphorescence.

The Light Centers of Organic Compounds

During recent years a very large number of investigations have been carried out concerning the nature of the absorption light centers of organic compounds, both pure and in a state of solution. These centers have been roughly defined as chromophores, the chromophores consistinoof radicles of the given compounds that are found necessary and sufficient to produce the given absorption. Among the chromophores that might be cited are ${\displaystyle {\ce {>\ C=C<\ ;=CO\ ;>\ C=NH,-N=O;=N=O;=C=S,}}}$ etc, A bathochrome introduced into an organic compound causes the absorption band to become wider. An auxochrome causes the intensity of the absorption to be greater.

Dynamic Isomerism

Baly and many others have supported the view that the absorption of many organic compounds is due to a change in the valency linking of a compound. This dynamic isomerism is known to take place in many chemical compounds in the presence of a catalytic agent or at high temperatures. Take the case of acetylacetone and ethyl acetoacetate. The absorption in this case may be due to a reaction changing the ketonic (1) into the enolic (2) form and some experimental evidence favors this view.

Résumé

From the above brief account of our knowledge concerning the nature of the absorption and emission centers of light and heat radiations it will be noted that many advances have been made toward the solution of this problem in recent years. The existence of "electron" atmospheres in many solids, liquids and gases has explained the emission and absorption of spectra that are ordinarily described as continuous; the existence of negative electrons serves to explain many phenomena such as those of the Zeeman effect, etc.; models containing elementary magnets arranged in various ways have been used by Ritz to explain the series classification of spectrum lines; the various phenomena of ionization are being found to be intimately correlated with the phenomena of light emission and much evidence is being accumulated to show that light and heat centers may ultimately be identified as consisting of certain kinds of ions; a very large amount of experimental data has been accumulated concerning absorption spectra of solutions of organic and inorganic compounds and the centers of this absorption seem to consist in certain "aggregates," "chromophores," etc., which can be studied from other points of view; much evidence is found to point to the view that light and heat centers depend upon certain dynamic conditions and are not stable systems such as we usually conceive atoms and molecules to be.