Emission spectrum

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(1) Two representations of the emission spectrum of a fluorescent lamp; the numbers indicate the wavelength (in nm) of the mercury spectral lines. Other bands do not have numbers - these are the emissions of the fluorescent substances excited by the UV radiation of the mercury plasma .
(2) Two representations of the spectrum of a low pressure mercury vapor lamp. Upper picture with a 256-pixel line sensor . Lower shot with a camera
(3) Two representations of the spectrum of a low-pressure cadmium vapor lamp. Upper picture with a 256-pixel line sensor . Lower shot with a camera

An emission spectrum is the electromagnetic spectrum that is emitted by atoms , molecules or materials without electromagnetic radiation of the same frequency being emitted . The counterpart of an emission spectrum is the absorption spectrum . While discrete energy levels produce a line spectrum , energy bands produce a continuous spectrum.

Atomic spectrum

An atomic spectrum , the emission spectrum of a single isolated atom , so the intensity of the light emitted by it ( emitted ) light as a function of wavelength (or frequency). The individual spectral lines of the spectrum correspond to the energy difference between two different states of the atom, which can be applied, for example, by an absorbed light particle ( photon ) and then emitted again in the form of another photon with that energy. This energy difference (or the line in the spectrum) is discrete , so it cannot assume any values, so that each atom - according to its specific electron configuration - can only emit photons of very specific, discrete wavelengths, which are characteristic of the individual chemical elements .

The fact that the “lines” in the spectrum (“peaks”) are precisely curves with a certain width is based on quantum effects. A comparison of the two images above (1, 2) shows that the spectral "lines" become wider with increasing gas pressure and the width of these "lines" can increase so much at very high pressure that one practically creates a light continuum similar to that of a radiant one Solid observed. The cause of this effect is the increasing frequency of mutual disturbances of the atoms when they collide with increasing pressure. The example of the spectral line at 491.6 nm clearly shows how strongly the relative intensity of this line depends on the respective gas pressure.

In the lower picture (3) one can see another characteristic of the spectral lines for the element cadmium.

If an electron is completely detached from its atom by absorption of a photon , this is called ionization of this atom, for which, depending on the initial state of the electron, a certain minimum energy is necessary. Energy supplies above this minimum energy always lead to ionization, which explains why, in contrast to what has been said so far, continua are possible in this energy range . The reverse process, i.e. H. The emission of a photon when an electron is captured by a positive ion is called recombination , which, as just explained, can also produce continuous radiation, the so-called. Boundary continuum .

Molecular spectrum

As a result of a spectrum, a molecular spectrum is the same as the atomic spectrum described. However, the energy levels are mostly different and so the relevant lines are in the infrared.

Emission spectrum of a solid or liquid material

While the emission spectrum of dilute gases results in a line spectrum , hot solids and liquids emit a continuous spectrum because the individual atoms also interact with each other and thus the discrete quantum states merge. Such a continuous spectrum can be calculated by multiplying the spectrum of a black body ( Planck's law of radiation ) of the same temperature by the absorption coefficient for electromagnetic radiation of the "object" at the respective wavelength.

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Wiktionary: emission spectrum  - explanations of meanings, word origins, synonyms, translations