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Width of spectral lines

  1. Mar 29, 2015 #1
    I have two questions regarding spectral lines, in the optical wavelengths.

    Every line has a width. One reason is the uncertinity in energy of the atomic states according to Heisenberg uncertinity relation. But this is just a very small part of the width, I think. Are there other causes to this, anything involving for example termo movements of the atoms?

    The other questions is about emission and absorption spectral lines. If, for example, heating a gas, the gas emitting photons of a wave length corresponding to the switch of energy states in the atoms. A typical time interval is about 10-8 sec for an atom being in excited state; after that time it fall back and emitting a photon, right? But when sending light into that gas, it absorbes the corresponding wavelengths, and it appears dark lines in the spectrum. The explation use to be that the atom absorbes the energy, but shouldn't it fall back and emit a photon of that wavelength after 10-8 sec?

    Thank you for your input!
  2. jcsd
  3. Mar 29, 2015 #2
    Yes to temperature widening the spectral lines! Especially temperature I believe!
    Dark line because emission can happen in any direction other than towards your eyes!
  4. Mar 31, 2015 #3


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    I believe this is the wrong way to look at it. The fact that lines have a certain width in frequency is a consequence of the fact that they have a finite lifetime. The fact that the width is inversely proportional to the lifetime is mathematical result and is true for every system (i.e. not only in physics) and is sometimes known as the mathematical uncertainty principle: it is if you want a direct consequence of the "definition" of frequency (in Fourier analysis).
  5. Mar 31, 2015 #4


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    The reason for the instability of all (atomic) states except the ground state is the coupling to the electromagnetic quantum field, leading to spontaneous emission of a photon and transition from an excited state to a lower-lying state. This is what's known as "natural line width". Spontaneous emission is one of the true quantum features of the electromagnetic field and was discovered by Dirac in 1928.
  6. Mar 31, 2015 #5


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    There are two sources of thermal broadening: Doppler broadening and collisional broadening. In normal conditions, thermal broadening will be much greater than that due to the natural linewidth.

    You usually are measuring the absorption along the path of the incident light, but the emission will be isotropic, so only a negligible fraction of the light re-emitted by the atoms will be detected.
  7. Mar 31, 2015 #6


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    Some other sources of line broadening are Doppler broadening, instrumental broadening, pressure broadening, and Stark broadening.
    Doppler broadening can be from random thermal motion, in which case it has Gaussian shape, or could be from looking at a beam, and then it depends on the aperture of the beam and the view. Instrument broadening is due to the limitations of your equipment. Pressure broadening has to do with collisions modifying the lifetime of the excited states. Stark broadening is actually Stark splitting, but if the resolution of your equipment isn't good enough, the split lines blend together and look like broadening.

    Ooops, DrClaude already said some of that.
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