Absorption line spectra and emission line spectra

[Zman]

Why don’t emission and absorption properties of an atom cancel out such that no such spectral lines are seen.

If electrons in atoms are excited from level 1 to level 2 and absorb energy at some specific frequency, why don’t they then emit that same frequency when they fall back down to level1 and cancel out the absorption lines with the emission lines.

Thanks

 

[Bob_for_short]

Imagine a dense gas cloud. You look at a star through it. The resonance photons are obsorbed in the gas and then the excited atoms/molecules collide and transform the excitation energy into heat. Then the cloud will absorb this spectral line.

If there is no non-radiative loss mechanisme, then the cloud will diffuse the absorbed light to all directions which is somewhat similar to partial absorption.

 

[Bob S]

Hi Zman-
I have looked at the sodium doublet (yellow)D-lines with a ruled grating spectrometer. The sodium arc is a lot hotter than the surrounding cool sodium gas. The cooler gas absorbs the emission lines in the center of each sodium line, so that the resultant sodium D-lines each have a narrow absorpton notch in the center.
Bob S

 

[alxm]

A valid question. In many cases they would, although it depends on what transition you're talking about, but anyhow.. In general, if you want to measure an emission spectrum, you do so at a 90 degree angle from your excitation source, so that those photons don't enter your detector. The re-emitted photons go in all directions.

 

[Zman]

Thank you for your helpful responses.

In general, if you want to measure an emission spectrum, you do so at a 90 degree angle from your excitation source, so that those photons don't enter your detector. The re-emitted photons go in all directions.

What would you see if placed the detector head on instead of 90 degrees.
Would the absorption lines just be reduced?



If a photon is absorbed by an atom, presumably the direction of the subsequent emission is not necessarily related to the original direction of the photon.
Is this emission direction random or maybe even like a diffraction pattern?
It would make sense that it was something like this from what has been said. The absorption spectra would seem to require it.

And yet with a mirror the incident direction of the photon is related to the emission direction of the photon. But presumably the photons here are not being absorbed and there is a different rule at work.

 

[Bob S]

What would you see if placed the detector head on instead of 90 degrees.
Would the absorption lines just be reduced?.

Hi Zman-
If the absorber were placed directly in between the (thermally hotter) source and the (colder) absorber, you would see a dark band in middle of the source emission line(s). It is very educational to look at the sodium D lines (about 5880 Angstroms) with a diffraction grating. You will see the two 3P to 3S lines, each with a dark absorptin band in the middle.
Bob S