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I was reading about pressure broadening for atomic spectra. What I can't understand is why it is important in absorption spectra if the collisions affect only the lifetime of the excited states.

It makes sense that if an atom is an excited state and collides with another atom then the lifetime of the excited state is reduced. So the emission lines should be broadened. How can we apply the same to absorption lines?

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In principle you gave the answer already. The excitation energy is uncertain because due to possible collisions (becoming more and more probable with increasing pressure) the average lifetime of the excited state decreases and so the energy level gets broader. As an effect, a broader sprectrum of electromagnetic radiation (photons of different energies) can be absorped.

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  • $\begingroup$ So it doesn't matter if the collision happens before the excitation right? $\endgroup$
    – Anton
    Aug 27, 2021 at 8:52
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    $\begingroup$ I guess it's a statistcal process happening all the time. If you want to break it down to a single atom you end up with probabilities, averages become expectation values etc. But what you normally observe is the absorption of an incredibly large number of atoms or molecules within some volume of gas. $\endgroup$ Aug 27, 2021 at 9:02

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