Question about absorption spectra

In summary, electrons can remain in their new orbit for a very long time. They can be knocked clear of the nucleus. They can jump to the other atom. The point is that the electron is unlikely to go straight back to its original level.
  • #1
toastisme
2
0
Been viewing this site for a while now and very much appreciate all the contributors! I can't seem to find an answer to this anywhere so I think I may be misunderstanding the basic ideas of absorption and emission spectra...
An absorption spectrum, say of our Sun, is as I understand it the result of electrons being promoted to higher energy levels through accepting photons at certain wavelengths, thus giving black lines in a spectrum. However, this energy is released as the electron 'wants' to return to a more stable state with lower energy (which gives an emission spectra if there isn't a light source behind the object). So my question is, how quickly do electrons emit this energy? As I understand it, it should be immediately, but wouldn't that then cancel out the absorption lines? Or is a spectrum only a 'snapshot', with spectra shown in real time having the lines fade in and out?

Edit: sorry just saw an answer to this towards the bottom of the page! If someone could delete this...
 
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Hi, welcome to PF! You make a good observation, it had never occurred to me before. I think you are right in presuming that an electron, after excitation, will sometimes immediately (or after a short delay) undergo de-excitation via the reverse process, i.e. between the same two atomic states, thus re-emitting a photon of the same energy as was absorbed. However, this photon is emitted in a random direction, whereas the white source beam is necessarily directional. Therefore, the beam becomes deficient of photons with energies corresponding to atomic transitions.

Another factor, although perhaps a minor one: after absorbing a photon, an atom may shed the acquired energy via any of a range of de-excitation paths. The simple case of de-excitation via the reverse process of one-photon excitation is just one of these paths. Others include double- or multiple-photon emission, or some more complex cascade of electronic transitions involving multiple electrons, and there are probably many other paths that I have not even heard of. These processes would not contribute to cancelling the absorption lines. However, I must admit I do not know how the branching ratio typically falls for these exotic de-excitation paths—they might be very rare. Perhaps someone with knowledge can enlighten us.
 
  • #3
toastisme said:
Been viewing this site for a while now and very much appreciate all the contributors! I can't seem to find an answer to this anywhere so I think I may be misunderstanding the basic ideas of absorption and emission spectra...
An absorption spectrum, say of our Sun, is as I understand it the result of electrons being promoted to higher energy levels through accepting photons at certain wavelengths, thus giving black lines in a spectrum. However, this energy is released as the electron 'wants' to return to a more stable state with lower energy (which gives an emission spectra if there isn't a light source behind the object). So my question is, how quickly do electrons emit this energy? As I understand it, it should be immediately, but wouldn't that then cancel out the absorption lines? Or is a spectrum only a 'snapshot', with spectra shown in real time having the lines fade in and out?

Edit: sorry just saw an answer to this towards the bottom of the page! If someone could delete this...
As I understand it, it should be immediately, but wouldn't that then cancel out the absorption lines?
This is not true at all. The electron can remain in its new orbit for a very long time. For atomic gases under low pressure, it could be microseconds. Then, it may go back to a different lower energy level.
There are collisions at higher pressures. The electron can be knocked clear of the nucleus. It can be knocked into a lower or higher energy level. It can jump to the other atom.
If the gas were made of molecules instead of atoms, the electron could relax to a different vibrational or rotational level. Then the molecule could vibrate or rotate.
The point is that the electron is unlikely to go straight back to its original level. An electron can have a big "hang time". All sorts of thing can happen to the electron other than going back to the original level.
On the sun, the big thing that alters spectra are the collisions between atoms and ions. I don't think an excited electron has much of a chance of getting home!
 

1. What is an absorption spectrum?

An absorption spectrum is a graph that shows the amount of light absorbed by a substance at different wavelengths. Different substances have unique absorption spectra due to their specific molecular structures.

2. How is an absorption spectrum measured?

An absorption spectrum is measured using a spectrophotometer, which measures the amount of light absorbed by a substance at different wavelengths. The data is then plotted on a graph to create the absorption spectrum.

3. What is the significance of absorption spectra in scientific research?

Absorption spectra are important in identifying and analyzing substances. They provide information about the molecular structure and composition of a substance, which can be useful in fields such as chemistry, biology, and environmental science.

4. How does the absorption spectrum of a substance relate to its color?

The absorption spectrum of a substance can determine its color. A substance appears a certain color because it absorbs all other colors of light and reflects the color that we see. The absorption spectrum shows which wavelengths of light are being absorbed and which are being reflected, thus determining the color of the substance.

5. Can absorption spectra be used to identify unknown substances?

Yes, absorption spectra can be used to identify unknown substances by comparing their spectra to known spectra of different substances. This can help scientists determine the composition and properties of the unknown substance.

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