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There are various absorption lines that correspond to the difference in energy levels between electron orbits. E.g. the https://en.wikipedia.org/wiki/Lyman-alpha_line correpsonding to the difference in energy between an electron orbiting a hydrogen atom in its ground state and the $n=2$ orbital.

But how "wide" are these lines? How large is the range of values that can cause a hydrogen atom to absorb these photons and excite the electron. And what happens to the energy above the bare minimum required?

Similarly, how wide are the emission lines?

Brendan Darrer
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blademan9999
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    There are relevant related answers here, like this one: https://physics.stackexchange.com/a/443060/313612. – Ed V Jul 16 '23 at 12:29
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    It's a complex and fascinating subject. https://en.wikipedia.org/wiki/Spectral_line_shape – John Doty Jul 16 '23 at 14:14

2 Answers2

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There are basically 3 broadening mechanisms for spectral lines

  1. natural broadening due the finite lifetime of atomic states (states that decay faster lead to broader lines),

  2. Doppler broadening due to the thermal velocities of the atoms,

  3. pressure/collisional broadening due to collisions of the radiating atom with other particles (whilst these collisions may not actually shorten the lifetime of the level, they introduce random phase jumps of the emitted wave, which have the same effect on the observed line width).

For more see http://www-star.st-and.ac.uk/~kw25/teaching/nebulae/lecture08_linewidths.pdf

Thomas
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  • There are others: proximity broadening, opacity broadening, ... – John Doty Jul 16 '23 at 14:52
  • @JohnDoty Proximity broadening is in practice only relevant for fluids and solids, and thus hardly for the spectral line series of atomic hydrogen. Opacity broadening is only a secondary effect due to radiative transfer effects (radiation being redistributed by scattering/absorption from the line center to the line wings) and does not affect the absorption/emission line profile of the atom as such.. – Thomas Jul 16 '23 at 15:23
  • These are both things that affect real spectra in the real world. They don't affect whiteboard abstract spectra that are rarely seen in reality. – John Doty Jul 16 '23 at 16:15
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    @JohnDoty I am trying to answer the OP's question. Proximity broadening has zero relevance for the spectra of atomic hydrogen in the real world. – Thomas Jul 16 '23 at 16:33
  • And I am trying to point out that if you only cover the textbook idealization, you're misleading the poster. – John Doty Jul 16 '23 at 16:39
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    @JohnDoty On the contrary, it would be misleading if I would make the OP believe that irrelevant or marginal physical effects are relevant for his particular question. It is called didactics. – Thomas Jul 16 '23 at 17:09
  • Thus, students only learn whiteboard cartoon physics, not the physics of the real world. – John Doty Jul 16 '23 at 23:18
  • @JohnDoty Students always only learn the basics. The fine details relevant for certain special applications are added when you are doing a Ph.D. thesis etc. Wikipedia is not always a good source to get a basic understanding of things. It is often written/edited by researchers trying to promote their own specialized view of the subject. – Thomas Jul 17 '23 at 07:18
  • Let us continue this discussion in chat. – John Doty Jul 17 '23 at 11:29
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I have seen line widths in the lab of about 100 MHz from the Doppler shift in an ion beam.

This article says that a transition in Cs has a natural linewidth of approximately 5 MHz. https://nopr.niscpr.res.in/bitstream/123456789/13997/1/IJPAP%2050%285%29%20295-298.pdf

Emission and absorption widths are going to be the same if the atoms in the gas have the same distribution of velocities.

Dr. Nate
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