Absorption spectrum

“ 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. ”

“ If no wave-lengths of the visible spectrum had been absorbed, we should have had the continuous spectrum of white light in the light reflected, i.e., the body itself would not be colored. Colored substances, therefore, absorb the rays of their complementary colors and, consequently, when white light is transmitted through them their spectra will indicate the regions of this absorption by dark bands of varying intensity. The absorption spectrum coincides always with the spectrum obtained from the reflected light. ”

“ With regard to the well-known statement that solids and liquids give continuous spectra, while gases give line-spectra, it must be remarked that metallic vapors show in nearly all cases a continuous spectrum before they condense. Oxygen gives a continuous spectrum at the lowest temperature at which it is luminous. Examining liquids and solids by the method of absorption, we find that many of them show discontinuous spectra, presenting fairly narrow bands. It is not denied that the nearness of molecules does not affect the spectrum. ”

“ ABSORPTION OF LIGHT. The term ``absorption'' (from Lat. absorbere) means literally ``sucking up'' or ``swallowing,'' and thus a total incorporation in something, literally or figuratively; it is technically used in animal physiology for the function of certain vessels which suck up fluids; and in light and optics absorption spectrum and absorption band are terms used in the discussion of the transformation of rays in various media.
If a luminous body is surrounded by empty space, the light which it emits suffers no loss of energy as it travels outwards. ”

“ The absorption spectrum of carbon, or that which should be composed of the dark lines detaching themselves upon a continuously bright spectrum, has not yet been obtained. In the comparative study that I have made of the solar spectrum and the spectrum of carbon, I have shown that most of the bright rays forming the carbon bands do not coincide with the dark rays of the solar spectrum. ”

“ On employing a very strong solution of the naphthaline coloring material, the whole spectrum vanishes with the exception of the red end, which remains apparent to a point behind C. If now the absorption spectrum be compared with that thrown upon the fluid, the intimate relation between absorption and fluorescence that has already been pointed out in the esculine solution is corroborated in the minutest particulars. For every dark hand in the absorption spectrum corresponds to a bright band in the fluorescing spectrum. ”

“ The spectra of this class show a singular mixture of bright lines and dark bands, as if three different spectra were combined, one continuous, one an absorption spectrum, and one an emission spectrum from glowing gas. Less than a hundred stars of this type have been discovered. ”

“ Ultimate lines of neutral atoms. Ordinates are logarithms of computed fractional concentrations; abscissae are temperatures in thousands of degrees. The curves show the decrease in the number of neutral atoms, with rising temperature, and the consequent decay in strength of the ultimate lines, for the atoms indicated on the right margin.
As ionization becomes more and more complete, the intensity of the ultimate lines falls off until so small a number of neutral atoms remains that their lines cease to appear in the absorption spectrum. ”

“ The fact that the gases with which we are most familiar are not rendered luminous by being heated in a tube to a temperature well above a white heat has often been a stumbling block and raised the not unreasonable doubt whether approximately homogeneous oscillations could ever be obtained by a mere thermal process. The experiment proves only the transparency of the gases experimented upon, and this is confirmed by the fact that bodies like bromine and iodine give on heating an emission spectrum corresponding to the absorption spectrum seen at ordinary temperatures. ”

“ The relative intensity of lines in the spark taken from metallic poles may be altered by the insertion of greater or smaller capacities, similarly the relative intensities are different in arc and spark spectra, (g) Increased pressure nearly always diminishes the frequency of vibration, but this effect is generally of a smaller order of magnitude than the widening which takes place in the other cases. In investigating the effects of mixture on the widening of lines in absorption spectrum, R. W. Wood discovered some interesting effects. ”

“ Since these are the two elements from which the separation is most difficult, and are moreover the most constant in their occurrence with samarium, the absorption spectrum is of very little use as a test.
The arc spectrum is very rich in lines, [299] of which the most intense are:
[299] Exner and Haschek; Eder and Valenta; Rütten and Mersch, Zeitsch. wiss. Photochem. ”

“ When a body, even if gaseous in form, is of such great size and density that light of no color can pass entirely through it, then the consequent absorption by the body of light of all colors shows that throughout the region where the absorption occurs there must be an emission of light of these same colors. Thus light from all parts of the spectrum will be emitted by the entire mass. ”

“ Thus prismatic analysis has no power to identify individual kinds of matter, except when they present themselves as glowing vapours.
A spectrum is said to be "reversed" when lines previously seen bright on a dark background appear dark on a bright background. In this form it is equally characteristic of chemical composition with the "direct" spectrum, being due to absorption, as the latter is to emission. ”

“ We are here manifestly dealing with that great principle which lies at the basis of spectrum analysis, and which has enabled scientific men to determine the substances of which the sun, the stars, and even the nebulae are composed; the principle, namely, that a body which is competent to emit any ray, whether of heat or light, is competent in the same degree to absorb that ray. The absorption depends on the synchronism existing between the vibrations of the atoms from which the rays, or more correctly the waves, issue, and those of the atoms on which they impinge. ”

“ When the retardation of these oscillation frequencies is continued, as, for example, by the introduction of heavier atomic complexes for the simpler and lighter hydrogen atoms, the absorption curves due to oscillation will gradually be made to travel toward the red end of the spectrum and the color, naturally, will travel into the blue. A very well-known example of this is the increase in depth of the blue color possessed by certain dyes which accompanies an increase in the number of methyl groups introduced into the molecule. ”

“ The “layer” of which we can obtain a spectrum is therefore not at the same depth for all frequencies; it is most deep-seated in regions of continuous background, and nearest to the surface of the star at the centers of strong absorption lines. The pressures from which the different parts of the spectrum originate differ in the same way, and the idea of “pressure in the reversing layer” is not an easy one to define significantly. ”

“ The reversing layer, comprising the layers above the photosphere, where the general opacity has greatly decreased and selective opacity begins to be appreciable, is responsible for the lines in the spectrum, which form the major part of the material of stellar spectroscopy. When the energy flowing out through the reversing layer in any specified wave-length is less than the energy in the neighboring continuous background, an absorption line is produced in the spectrum. ”

“ Because calculation of the absorbance area involves many points in the sample spectrum, Equation C3 leads to much more accurate results than the single-point calculation represented by Equation C2.
However, when many absorbing compounds are present in a sample, the absorption patterns of the various compounds often overlap. In this case, there is usually not an isolated analytical region for each compound in which only that compound absorbs infrared radiation; no single absorbance point and no simple absorbance area is suitable for determining any of the component concentrations. ”

“ For example, we have seen that double linkings in themselves do not possess any power for light absorption. Their mere presence, therefore, can not account for color in a chemical compound, but if by their presence some form of oscillation is produced, we may expect the establishment of definite vibrations in the ether, which will be possible of detection in the spectrum. In compounds of the quinoid type the conditions are precisely those that will produce vibrations in the ether corresponding in wave-length to portions of the visible spectrum, consequently the appearance of color. ”

“ These bands appear in the solar spectrum as we observe it, but are due to absorption by the oxygen contained in the atmosphere.
If oxygen is rendered luminous by the electric discharge, a series of spectra may be made to appear. Under different conditions we obtain (a) a continuous spectrum most intense in the yellow and green, (b) the spectrum dividing itself into two families of series, (c) a spectrum of lines which appears when a strong spark passes through oxygen at atmospheric pressure, (d) a spectrum of bands seen in the kathode glow. ”

“ This kind of absorption is produced by the vapours of metalloids or of compound substances.
To the fourth order of stars belongs also a colonnaded spectrum, but reversed; the light is thrown the other way. The three broad zones of absorption which interrupt it are sharp towards the red, insensibly gradated towards the violet end. The individuals composing Class IV. are few and apparently insignificant, the brightest of them not exceeding the fifth magnitude. They are commonly distinguished by a deep red tint, and gleam like rubies in the field of the telescope. ”

“ Compounds generally show spectra of resolvable bands, and
if an elementary body shows a spectrum of the same type we are probably justified in assuming it to be due to a complex molecule. But that it may be given by the ordinary diatomic molecule is exemplified by oxygen, which gives in thick layers by absorption one of the typical sets of Bands which were used by Deslandres and others to investigate the laws of distribution of frequencies. ”

“ The author has observed a fact which furnishes a remarkable demonstration of the law of the production of the dark bands which he has detected in the spectrum of oxygen. The phenomena of elective absorption in oxygen gas are manifested in two mutually distinct spectral systems. A first system, formed of fine rays, follows the law of the product of the gaseous system traversed by its density. The second system is formed of bands much less easily resolved, is governed by the law of the product of the thickness by the square of the density. ”

“ So we may have both the weak but complete spectrum produced by the light reflected from the outer strata in which the absorption has been insensible, and another spectrum coming from the deeper parts, with which the absorption has been greater. This view is confirmed by M. Respighi's spectroscopic observations of comet b, 1881. ”

“ Our green ribbon in the red of the spectrum quenched the red, but its temperature at the same time was augmented to a degree exactly equivalent to the light extinguished. Our black ribbon, when passed through the spectrum, was found competent to quench all its colours; but at every stage of its progress an amount of heat was generated in the ribbon exactly equivalent to the light lost. It is only when absorption takes place that heat is thus produced: and heat is always a result of absorption. ”

“ Though few follow him so far, an explanation Anomalous Dispersion. of the principle will make it clear that there are numerous possible opportunities for anomalous dispersion to qualify inferences from the spectrum. Theoretically anomalous dispersion is inseparable from absorption. When a system vibrating in a free period of its own encounters, say through the medium of an enveloping aether, a second system having a different free period, and sets it in vibration, the amplitude of the second vibration is inconsiderable, except when the periods approach equality. ”

“ As the origin of absorption bands in the ultra-violet spectrum have received an explanation in the change of linking brought about by the shifting of a labile atom, so clearly represented in the examples of keto-enol tautomerism, we may rightly expect to find absorption bands in the spectra of other compounds in which some change of linking is exhibited. No more beautiful example can be found than that of the compound known as benzol, where six carbon atoms, unchangeable in their order, are bound together in a single ring. ”

“ Considering the infinite variety of light, and its easy modification and absorption, we should expect an immense number of spectra. A mere prism disperses the light so imperfectly that different orders of vibrations, perceived as colors, are mingled. No eye can tell where one commences or ends. Such a spectrum is said to be impure. What we want is that each point in the spectrum should be made of rays of the same number of vibrations. As we can let only a small beam of light pass through the prism, in studying celestial objects with a telescope and spectroscope we must, in
Fig. ”