Emission spectrum
The emission spectrum of a
Emission
In physics, emission is the process by which a higher energy quantum mechanical state of a particle becomes converted to a lower one through the emission of a photon, resulting in the production of light. The frequency of light emitted is a function of the energy of the transition.
Since energy must be conserved, the energy difference between the two states equals the energy carried off by the photon. The energy states of the transitions can lead to emissions over a very large range of frequencies. For example,
The emittance of an object quantifies how much light is emitted by it. This may be related to other properties of the object through the Stefan–Boltzmann law. For most substances, the amount of emission varies with the
Emission of radiation is typically described using semi-classical quantum mechanics: the particle's energy levels and spacings are determined from quantum mechanics, and light is treated as an oscillating electric field that can drive a transition if it is in resonance with the system's natural frequency. The quantum mechanics problem is treated using time-dependent perturbation theory and leads to the general result known as Fermi's golden rule. The description has been superseded by quantum electrodynamics, although the semi-classical version continues to be more useful in most practical computations.
Origins
When the
The fact that only certain colors appear in an element's atomic emission spectrum means that only certain frequencies of light are emitted. Each of these frequencies are related to energy by the formula:
The frequencies of light that an atom can emit are dependent on states the electrons can be in. When excited, an electron moves to a higher energy level or orbital. When the electron falls back to its ground level the light is emitted.
The above picture shows the visible light emission spectrum for hydrogen. If only a single atom of hydrogen were present, then only a single wavelength would be observed at a given instant. Several of the possible emissions are observed because the sample contains many hydrogen atoms that are in different initial energy states and reach different final energy states. These different combinations lead to simultaneous emissions at different wavelengths.
Radiation from molecules
As well as the electronic transitions discussed above, the energy of a molecule can also change via
Emission spectroscopy
Light consists of electromagnetic radiation of different wavelengths. Therefore, when the elements or their compounds are heated either on a flame or by an electric arc they emit energy in the form of light. Analysis of this light, with the help of a
The emission spectrum can be used to determine the composition of a material, since it is different for each element of the periodic table. One example is astronomical spectroscopy: identifying the composition of stars by analysing the received light. The emission spectrum characteristics of some elements are plainly visible to the naked eye when these elements are heated. For example, when platinum wire is dipped into a sodium nitrate solution and then inserted into a flame, the sodium atoms emit an amber yellow color. Similarly, when indium is inserted into a flame, the flame becomes blue. These definite characteristics allow elements to be identified by their atomic emission spectrum. Not all emitted lights are perceptible to the naked eye, as the spectrum also includes ultraviolet rays and infrared radiation. An emission spectrum is formed when an excited gas is viewed directly through a spectroscope.
Emission spectroscopy is a
There are many ways in which atoms can be brought to an excited state. Interaction with electromagnetic radiation is used in
Although the emission lines are caused by a transition between quantized energy states and may at first look very sharp, they do have a finite width, i.e. they are composed of more than one wavelength of light. This
Emission spectroscopy is often referred to as optical emission spectroscopy because of the light nature of what is being emitted.
History
In 1756 Thomas Melvill observed the emission of distinct patterns of colour when
In 1835, Charles Wheatstone reported that different metals could be distinguished by bright lines in the emission spectra of their sparks, thereby introducing an alternative to flame spectroscopy.[6][7] In 1849, J. B. L. Foucault experimentally demonstrated that absorption and emission lines at the same wavelength are both due to the same material, with the difference between the two originating from the temperature of the light source.[8][9] In 1853, the
In 1854 and 1855,By 1859,
Experimental technique in flame emission spectroscopy
The solution containing the relevant substance to be analysed is drawn into the burner and dispersed into the flame as a fine spray. The solvent evaporates first, leaving finely divided
On a simple level, flame emission spectroscopy can be observed using just a flame and samples of metal salts. This method of qualitative analysis is called a flame test. For example, sodium salts placed in the flame will glow yellow from sodium ions, while strontium (used in road flares) ions color it red. Copper wire will create a blue colored flame, however in the presence of chloride gives green (molecular contribution by CuCl).
Emission coefficient
Emission coefficient is a coefficient in the power output per unit time of an
Scattering of light
In Thomson scattering a charged particle emits radiation under incident light. The particle may be an ordinary atomic electron, so emission coefficients have practical applications.
If X dV dΩ dλ is the energy scattered by a volume element dV into solid angle dΩ between wavelengths λ and λ + dλ per unit time then the Emission coefficient is X.
The values of X in Thomson scattering can be predicted from incident flux, the density of the charged particles and their Thomson differential cross section (area/solid angle).
Spontaneous emission
A warm body emitting
See also
- Absorption spectroscopy
- Absorption spectrum
- Atomic spectral line
- Electromagnetic spectroscopy
- Gas-discharge lamp, Table of emission spectra of gas discharge lamps
- Isomeric shift
- Isotopic shift
- Luminous coefficient
- Plasma physics
- Rydberg formula
- Spectral theory
- The Diode equation includes the emission coefficient (which is not related to the one discussed here)
- Thermionic emission
References
- ^ Incorporated, SynLube. "Spectroscopy Oil Analysis". www.synlube.com. Retrieved 2017-02-24.
- ^ Melvill, Thomas (1756). "Observations on light and colours". Essays and Observations, Physical and Literary. Read Before a Society in Edinburgh. 2: 12–90. ; see pp. 33–36.
- ^ See:
- Frauhofer. Jos. (1821) "Neue Modifikation des Lichtes durch gegenseitige Einwirkung und Beugung der Strahlen, und Gesetze derselben" (New modification of light by the mutual influence and the diffraction of [light] rays, and the laws thereof), Denkschriften der Königlichen Akademie der Wissenschaften zu München (Memoirs of the Royal Academy of Science in Munich), 8: 3–76.
- Fraunhofer, Jos. (1823) "Kurzer Bericht von den Resultaten neuerer Versuche über die Gesetze des Lichtes, und die Theorie derselben" (Short account of the results of new experiments on the laws of light, and the theory thereof) Annalen der Physik, 74(8): 337–378.
- PMID 16849159.
- ^ OpenStax Astronomy, "Spectroscopy in Astronomy". OpenStax CNX. Sep 29, 2016 http://cnx.org/contents/1f92a120-370a-4547-b14e-a3df3ce6f083@3
- ISBN 978-0-85296-103-2.
- ^ Wheatstone (1836). "On the prismatic decomposition of electrical light". Report of the Fifth Meeting of the British Association for the Advancement of Science; Held at Dublin in 1835. Notices and Abstracts of Communications to the British Association for the Advancement of Science, at the Dublin Meeting, August 1835. London, England: John Murray. pp. 11–12.
- ^ a b Brand, pp. 60-62
- ^ See:
- Foucault, L. (1849). "Lumière électrique" [Electric light]. Société Philomatique de Paris. Extraits des Procès-Verbaux de Séances. (in French). 13: 16–20.
- Foucault, L. (7 February 1849). "Lumière électrique" [Electric light]. L'Institut, Journal Universel des Sciences (in French). 17 (788): 44–46.
- ^ See:
- Ångström, A.J. (1852). "Optiska undersökningar" [Optical investigations]. Kongliga Vetenskaps-Akademiens Handlingar [Proceedings of the Royal Academy of Science] (in Swedish). 40: 333–360.
- Ångström, A.J. (1855a). "Optische Untersuchungen" [Optical investigations]. Annalen der Physik und Chemie (in German). 94: 141–165.
- Ångström, A.J. (1855b). "Optical researches". Philosophical Magazine. 4th series. 9: 327–342. .
- .
- ^ (Ångström, 1852), p. 352; (Ångström, 1855b), p. 337.
- .
- ^ See:
- Alter, David (1854). "On certain physical properties of light, produced by the combustion of different metals, in the electric spark, refracted by a prism". The American Journal of Science and Arts. 2nd series. 18: 55–57.
- Alter, D. (1855). "On certain physical properties of the light of the electric spark, within certain gases, as seen through a prism". The American Journal of Science and Arts. 2nd series. 19: 213–214. Alter's observations of hydrogen's optical spectrum appear on p. 213.
- ^ See:
- Gustav Kirchhoff (1859) "Ueber die Fraunhofer'schen Linien" (On Fraunhofer's lines), Monatsbericht der Königlichen Preussische Akademie der Wissenschaften zu Berlin (Monthly report of the Royal Prussian Academy of Sciences in Berlin), 662–665.
- Gustav Kirchhoff (1859) "Ueber das Sonnenspektrum" (On the sun's spectrum), Verhandlungen des naturhistorisch-medizinischen Vereins zu Heidelberg (Proceedings of the Natural History / Medical Association in Heidelberg), 1 (7) : 251–255.
- .
- .
- ISBN 978-0-8053-0402-2.