Neal
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Stokes vs. anti-Stokes shifts.
What are the criteria that determines whether an emission from an absorption, will be Stokes or anti-Stokes?
Stokes = absorb shorter wavelength -> emit longer wavelength.
Example: absorb UV, emit IR.
Anti-Stokes = absorb longer wavelength --> emit shorter wavelength.
Example: absorb IR, emit UV.
Are there 2 main variables, which are electronic transitions and thermal radiation?
And so liquid oxygen is anti-Stokes? (If red light is absorbed , then blue light is transmitted or reflected and this gives rise to the blue color
associated with liquid oxygen.).
I've heard that things that emit anti-Stokes, are still 1% anti-Stokes, 99% Stokes shifts. Also increasing the temperature increases the % of
anti-Stokes, but then what are some things that naturally have a higher % of anti-Stokes? Compton scattering (in the clouds)?
Edit: anti-Stokes has to do with where the photon originated from. It has to originate from a non-ground state. If a photon originated from a ground
state, then it can never do anti-Stokes.
[Edited on 20-10-2022 by Neal]
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Neal
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This is ironically the turning point question, that got me permanently banned at the Chemistry Stack Exchange forum. Anyone have an account there?
They shut it down for the question was "not focused" enough. And then started shutting down all my prior threads like why can't we add ozone to the
ozone layer. Smh.
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j_sum1
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I thought it was an interesting question and not at all vague. I did not reply because I have no knowledge of the subject. In fact, I had not even
heard of anti-Stokes fluorescence.
I was able to read up a little on it. Very interesting. Thanks for asking the question.
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Neal
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Going on to what I said about the 2 main variables being electronic transitions and thermal radiation. Thermal radiation being the kinetic energy
(translation, rotation, etc)..
Then this was my question, on determining whether the shift will be Stokes or anti-Stokes.
So, therefore, I find all these 9 combinations, what are examples where these are found in nature or in a lab?
Absorbing IR/UV, and
1a. Where thermal radiation is near 0, and electronic transitions is a big positive?
1b. Where thermal radiation is near 0, and electronic transitions is a big negative?
1c. Where thermal radiation is a big positive, and electronic transitions is near 0?
1d. Where thermal radiation is a big negative, and electronic transitions is near 0?
1e. Where both thermal radiation and electronic transitions are a big positive?
1f. Where both thermal radiation and electronic transitions are a big negative?
1g. Where both thermal radiation and electronic transitions are near 0?
1h-i. Where they are both in big opposite numbers, which both net to near 0.
Since anti-Stokes is rare, I'm guessing there is a lot of (big positive) and (smaller negative) that leads to the regular, thus making anti-Stokes
rare. So are any of the above 9 questions not found in nature?
Now, I can think of 2 natural examples. Clouds absorb IR and instantly release it at the same wavelength. For UV, plants are good UV absorbers,
without emitting much, and, snow is a good reflector of UV. But I'm assuming the % plants emit and the % snow reflects are similar wavelength UV it
absorbs.
Now, my original question was
"what are examples where something absorbs UV, but emits IR" or "absorbs IR, but emits UV" (anti-Stokes). But asking those led me to get into the
above 9 questions variables.
This was a response I got.
OK, I think you don’t understand how absorption works and so are making it much more complicated.
Molecules in general have 4 ways of storing energy, electronic, vibrational, rotational, and translational. Translational energy levels are so small
we don’t really worry about them. Rotational modes absorb in the microwave region, we’re not going to worry about them either right now.
Vibrational modes absorb in the IR and electronic absorbs in the UV-vis region. Again this is general, exceptions exist. Vibrations are the molecule
stretching and bending. That takes energy, and usually corresponds to a photon with an IR wavelength. Electronic modes are electrons being excited to
higher energy states, and that energy gap is usually in the UV or visible range.
Now the energy gap between electronic states is much larger than between vibrational states. So each electronic state has vibrational states within
it. So you have the ground electronic state with a ground vibrational state, then ground electronic and first vibrational excited state, and so on.
Then you have the first electronic excited state with its vibrational levels (and to go deeper each vibrational state has a set of rotational levels,
but that’s not important right now).
So when you electronically excite an electron it goes up an electronic energy level. But it doesn’t necessarily go into the same vibrational level
it came from. This is because the excited state will have the same inter-nuclear distance but a slightly different energy landscape so the same
vibration isn’t available. So it goes into a higher vibrational level. Then the electron can relax non-radiatively into lower vibrational levels
before de-exciting back to the ground electronic state. And again it may not drop back to the same vibrational state for the same reason as above.
So the result is due to vibrational relaxation the de-excitation energy gap is smaller than the excitation and so you get a lower energy photon. You
can also de-excite to a lower vibrational level and so have a shorter wavelength but this is rarer because you’d have to start in a non-ground
vibrational state which is statistically less probable from the Boltzmann distribution.
As for what molecules absorb in what modes, well that gets into selection rules which I’m not going to get into. There has to be a net change in
dipole moment for the transition to occur. If you want to get into that you’ll need a spectroscopy or Pchem text. Honestly if you want to get more
into absorption you’ll need to get into Pchem, it’s IMO really cool but also very complex. I’ve barely scratched the surface here.
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