Emission lines have a certain natural width. Due to the uncertainty principle systems that spontaneously decay or produce radiation have a fundamental energy blur, and their radiation has a corresponding natural line width.

I am wondering how that phenomenon translates to absorption lines?


1 Answer 1


The natural linewidth also causes absorption lines to be broadened in exactly the same way.

Usually, the natural linewidth is far narrower than the width caused by (i) Doppler broadening by thermal motions of the atoms/molecules, (ii) collisional broadening caused by interactions between atoms/molecules, (iii) the broadening imposed by the finite spectral resolution of the instrument being used to measure the spectrum.

  • 2
    $\begingroup$ Collisional broadening does not lead to a Gaussian profile but also a Lorentzian profile (effectively, it reduces the natural lifetime of the state). So it would not be distinguishable from natural broadening as such. See e.g. www-star.st-and.ac.uk/~kw25/teaching/nebulae/… for more details $\endgroup$
    – Thomas
    Commented Nov 15, 2022 at 21:43
  • $\begingroup$ @Thomas agreed. $\endgroup$
    – ProfRob
    Commented Nov 16, 2022 at 9:37

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