Post by: theThinker on 02/04/2023 00:19:44
1) what is the physical mechanism?
2) where will the energy go to?
Post by: Origin on 02/04/2023 00:40:04
Post by: theThinker on 02/04/2023 06:04:41
Photons are typically absorbed by an electron when interacting with atoms. The electron that absorbs a photon will move to a higher energy orbital.But there is the believe that an atom may absorb a photon only if the frequency matches the electron orbital levels. Absorption spetrum == emission spectrum, etc
Post by: alancalverd on 02/04/2023 07:42:40
Post by: evan_au on 02/04/2023 11:21:27
Quote from: TheThinker
there is the believe that an atom may absorb a photon only if the frequency matches the electron orbital levels. Absorption spetrum == emission spectrum, etcA precise line spectrum only applies to isolated atoms (eg in a rarified gas).
In metals (plus liquids and other solids), the atoms are tightly packed, and the Pauli Exclusion principle comes into play:
- No two interacting electrons can have exactly the same quantum state
- The precise electron energy levels you see in a gas broadens out into a wide band of electron energies in a solid
- So a photons with a wide range of energies will find an electron able to change levels by that amount
- Allowing photons of many energy levels to interact with the material.
There are many factors which contribute to spectral broadening (bored chemist must deal with all of these)
https://en.wikipedia.org/wiki/Spectral_line#Broadening_due_to_local_effects
But most metals are reflective - incoming photons bounce off, rather than being absorbed.
- Until the photon gets into the UV range, when it is possible to eject an electron from the surface, through the Photo-Electric effect (which won Einstein a Nobel Prize)
https://en.wikipedia.org/wiki/Photoelectric_effect
Post by: Origin on 02/04/2023 14:36:27
But there is the believe that an atom may absorb a photon only if the frequency matches the electron orbital levels.That is true, but my explanation is still correct. If the photon is below the necessary energy to raise the electron to a higher energy orbital the photon will not be absorbed. One thing I did not mention is that if the energy of the photon is very high then when an electron absorbs the photon the electron will be ejected from the atom. In the case where the electron is ejected from the atom there is no need for a specific energy level of the photon, as long as the energy is above the level needed to eject the electron. The energy in excess of the amount needed to remove the electron from the bound state will be seen in the kinetic energy of the now free electron.
Post by: theThinker on 02/04/2023 15:36:42
OK. I think I got it. It is too complicated for my understanding.Quote from: TheThinkerthere is the believe that an atom may absorb a photon only if the frequency matches the electron orbital levels. Absorption spetrum == emission spectrum, etcA precise line spectrum only applies to isolated atoms (eg in a rarified gas).
In metals (plus liquids and other solids), the atoms are tightly packed, and the Pauli Exclusion principle comes into play:
- No two interacting electrons can have exactly the same quantum state
- The precise electron energy levels you see in a gas broadens out into a wide band of electron energies in a solid
- So a photons with a wide range of energies will find an electron able to change levels by that amount
- Allowing photons of many energy levels to interact with the material.
There are many factors which contribute to spectral broadening (bored chemist must deal with all of these)
https://en.wikipedia.org/wiki/Spectral_line#Broadening_due_to_local_effects
But most metals are reflective - incoming photons bounce off, rather than being absorbed.
- Until the photon gets into the UV range, when it is possible to eject an electron from the surface, through the Photo-Electric effect (which won Einstein a Nobel Prize)
https://en.wikipedia.org/wiki/Photoelectric_effect
Say I have one microwave photon hitting an isolated air molecule (oxygen or nitrogen), what is the chances that the photon would be absorbed. How could we account with some rigor the changes in energy, before and after.
Post by: Bored chemist on 02/04/2023 15:46:03
Say I have one microwave photon hitting an isolated air molecule (oxygen or nitrogen), what is the chances that the photon would be absorbed.Zero
Oxygen and nitrogen are symmetrical and have no dipole moment, so the electric field of thephoton has nothing to grab hold of.
If you had a molecule of water vapour, there would be some chance of the microwave setting the water molecule spinning.
It's not usually thought of in terms of the chance of a photon being absorbed, but the fraction of a bunch of photons absorbed by a given thickness of the absorber.
If you do the maths you end up with a "capture cross section" which is the effective area that a molecule would need to have if it has a 100% chance of catching any photon that hit it.
Post by: Eternal Student on 02/04/2023 22:29:03
I've had three attempts at replying. I think the key is just to keep it really short and do only one thing. let's try this one thing.
Depends on the energy/frequency of the photon. Low frequencies (radio waves) will ... (do this), higher frequencies will .... (do that)...
No. The OP said "photon", they did not ask about "some radiation" or "an e-m wave". Therefore we can't really talk about classical models of e-m waves and how they interact with matter. A "photon" is a QM object.
There are many ways a classical e-m wave could interact with matter, however, there is only one model of interaction between a photon and some matter that is well understood. That's a Quantum mechanical phenomena and much as was described in one of the first posts (@Origin).
The fine details can vary but the basic theme is always the same:
We have some QM particles (they are usually electrons) that can exist in only a certain set of states, a photon is incident upon that and it could move one of those QM objects from one state to another.
Origin discussed the situation where those states were associated with orbitals around an atom. That's the main or vanilla flavoured situation and the best one to discuss. A common variation on the theme is a metal which has a delocalised conduction band. Same basic situation, you have some electrons and some states they can be in..... it's just that some of states are not associated with just one atom, they are delocalised AND also some states have high degeneracy (many states with the same energy) and the states can be very closely spaced - when all of these things happen we have the conduction band that is effectively a continuous band of energies that are possible and can accommodate many electrons etc.
- - - - - - - - - - -
@theThinker - did you really want to discuss just photons, or are you prepared to consider a more generalised electromagnetic wave that can be treated classically?
- - - - - - - - - - -
@theThinker also asked "why" or what the physical mechanism is. One answer that can be given is just that Quantum Mechanics (QM) does not tell you. We know that energy states for electrons are quantised, we know that they can change states and emit or absorb a photon in that process, we do not ask why. To make a more concrete demonstration of the inability to explain "why" or what the mechanism is you only need to consider our inability to predict WHEN it will happen. We can have an atom in an excited state, QM will tell us that it CAN emit a photon and drop to a lower energy state but it does not determine WHEN that will happen. You have all the time before it actually happens to ask yourself why it hasn't happened yet when everything seems to be set and ready to go.
OK.... that seems to be about three different things I might have mentioned, sorry.
Best Wishes.
Post by: theThinker on 03/04/2023 01:00:08
Hi.I am asking about a photon, not an em wave hitting matter.
I've had three attempts at replying. I think the key is just to keep it really short and do only one thing. let's try this one thing.Depends on the energy/frequency of the photon. Low frequencies (radio waves) will ... (do this), higher frequencies will .... (do that)...
No. The OP said "photon", they did not ask about "some radiation" or "an e-m wave". Therefore we can't really talk about classical models of e-m waves and how they interact with matter. A "photon" is a QM object.
There are many ways a classical e-m wave could interact with matter, however, there is only one model of interaction between a photon and some matter that is well understood. That's a Quantum mechanical phenomena and much as was described in one of the first posts (@Origin).
The fine details can vary but the basic theme is always the same:
We have some QM particles (they are usually electrons) that can exist in only a certain set of states, a photon is incident upon that and it could move one of those QM objects from one state to another.
Origin discussed the situation where those states were associated with orbitals around an atom. That's the main or vanilla flavoured situation and the best one to discuss. A common variation on the theme is a metal which has a delocalised conduction band. Same basic situation, you have some electrons and some states they can be in..... it's just that some of states are not associated with just one atom, they are delocalised AND also some states have high degeneracy (many states with the same energy) and the states can be very closely spaced - when all of these things happen we have the conduction band that is effectively a continuous band of energies that are possible and can accommodate many electrons etc.
- - - - - - - - - - -
@theThinker - did you really want to discuss just photons, or are you prepared to consider a more generalised electromagnetic wave that can be treated classically?
- - - - - - - - - - -
@theThinker also asked "why" or what the physical mechanism is. One answer that can be given is just that Quantum Mechanics (QM) does not tell you. We know that energy states for electrons are quantised, we know that they can change states and emit or absorb a photon in that process, we do not ask why. To make a more concrete demonstration of the inability to explain "why" or what the mechanism is you only need to consider our inability to predict WHEN it will happen. We can have an atom in an excited state, QM will tell us that it CAN emit a photon and drop to a lower energy state but it does not determine WHEN that will happen. You have all the time before it actually happens to ask yourself why it hasn't happened yet when everything seems to be set and ready to go.
OK.... that seems to be about three different things I might have mentioned, sorry.
Best Wishes.
My specific interest is whether a single microwave photon has a chance to be absorbed if it hits an isolated oxygen or nitrogen molecule or CO2 molecule. Boredchemist says no.
Post by: Bored chemist on 03/04/2023 08:55:23
My specific interest is whether a single microwave photon has a chance to be absorbed if it hits an isolated oxygen or nitrogen molecule or CO2 molecule. Boredchemist says no.And, give or take the possibility of a "not quite zero probability" due to isotopic substitution, he's sticking to it.
That's why we can use microwave radio signals to send data like wifi through air.
Post by: theThinker on 03/04/2023 16:12:46
OK. Accepted that wifi is the proof.My specific interest is whether a single microwave photon has a chance to be absorbed if it hits an isolated oxygen or nitrogen molecule or CO2 molecule. Boredchemist says no.And, give or take the possibility of a "not quite zero probability" due to isotopic substitution, he's sticking to it.
That's why we can use microwave radio signals to send data like wifi through air.
Post by: theThinker on 03/04/2023 16:14:19
1) How is the photon absorbed by the copper (ignore re-emission)
2) What actually happen when we say the temperature of the copper has risen.
Post by: Eternal Student on 03/04/2023 17:34:49
Boredchemist says no.He said a bit more than that. He's a chemist and it has been based on practical stuff rather than any theory. It's apparent the abosrption phenomena being described used a classical model of e-m radiation in its derivation. So there are some aspects that I just wouldn't be able to go along with. However, @Bored chemist has qualified his comments in a later post.
The main issue is that you ( @theThinker ) seem determined to consider a single photon. That's an extreme situation and therefore not really subject to classical models. However, you would have enormous amounts of trouble trying to detect anything with any equipment that exists. Even the most sensitive pices of equipment used in high energy laser laboratories and state of the art optics research don't reliably detect single photons. As for how you could detect the change that may have occurred in a single atom - well that again is just theory not something you would do in practice.
If you want just some rough practical rule, then nothing happens. You don't seem to be too interested in complex theory.
Let's just put a few minor points down here:
1. You ( @theThinker ) have started by assuming you will know the frequency of the photon you sent in. However, you just can't. There is an uncertainity principle relating energy and time. Depending on the way in which you generated the photon the uncertainty is usually spread out between the time of emission and the frequecy that it has. To say this another way, the photon does not have a well defined single frequency until after the atom has absorbed it. What you may think was a microwave photon is actually a superposition of many states and the best you could hope for is that the coefficients for the states were strongly peaked around the frequency, ν, that you think it should have. So a small but non-zero probality exists that the photon would act as if was in the visible light frequencies (or somewhere else).
2. A similar sort of uncertainty exists for the atom. You cannot know precisely what momentum it has, simply because you have needed to confined it to some region of space where you intend to aim the photon. If it is receeding from the photon then the effective frequency of the photon is red-shifted, conversely it would be blue-shifted if the atom was moving toward the photon at the time of emission. The more you confine the atom, to be sure that the photon is on target and likely to interact with it, the less you will know about the momentum of that atom.
3. Other stuff but you probably aren't too interested in the theory and I'm out of time for now.
Best Wishes.
Post by: theThinker on 03/04/2023 18:02:48
Hi.I do accept the comments you make about my assumptions - single photon, atom, etc.. But this is what most elementary textbook would assume.Boredchemist says no.He said a bit more than that. He's a chemist and it has been based on practical stuff rather than any theory. It's apparent the abosrption phenomena being described used a classical model of e-m radiation in its derivation. So there are some aspects that I just wouldn't be able to go along with. However, @Bored chemist has qualified his comments in a later post.
The main issue is that you ( @theThinker ) seem determined to consider a single photon. That's an extreme situation and therefore not really subject to classical models. However, you would have enormous amounts of trouble trying to detect anything with any equipment that exists. Even the most sensitive pices of equipment used in high energy laser laboratories and state of the art optics research don't reliably detect single photons. As for how you could detect the change that may have occurred in a single atom - well that again is just theory not something you would do in practice.
If you want just some rough practical rule, then nothing happens. You don't seem to be too interested in complex theory.
Let's just put a few minor points down here:
1. You ( @theThinker ) have started by assuming you will know the frequency of the photon you sent in. However, you just can't. There is an uncertainity principle relating energy and time. Depending on the way in which you generated the photon the uncertainty is usually spread out between the time of emission and the frequecy that it has. To say this another way, the photon does not have a well defined single frequency until after the atom has absorbed it. What you may think was a microwave photon is actually a superposition of many states and the best you could hope for is that the coefficients for the states were strongly peaked around the frequency, ν, that you think it should have. So a small but non-zero probality exists that the photon would act as if was in the visible light frequencies (or somewhere else).
2. A similar sort of uncertainty exists for the atom. You cannot know precisely what momentum it has, simply because you have needed to confined it to some region of space where you intend to aim the photon. If it is receeding from the photon then the effective frequency of the photon is red-shifted, conversely it would be blue-shifted if the atom was moving toward the photon at the time of emission. The more you confine the atom, to be sure that the photon is on target and likely to interact with it, the less you will know about the momentum of that atom.
3. Other stuff but you probably aren't too interested in the theory and I'm out of time for now.
Best Wishes.
I have always assumed a monochromatic light has photon of a definite frequency w/o uncertainty principle involved.
Post by: alancalverd on 03/04/2023 19:56:38
Post by: Eternal Student on 03/04/2023 22:42:59
But this is what most elementary textbook would assume.Which textbook? It would help to see what they said, or at least which topic they are covering and at what sort of level of study. Many school level textbooks would only mention a "photon" for the situation of raising an electron from one orbit to another (this will later be described as an atomic absorption). Meanwhile, a textbook for biochemists wishing to identify molecules by their infra-red absorption spectrum is referring to something different - in that situation you aren't really looking at "atomic absorption spectrums" but "molecular absorption spectrums". For the first (atomic absoption spectrums) you have electrons being directly moved to higher energy orbits. For the second (molecular absorption spectrums) you have much larger structures - molecules - and they are being made to vibrate or rotate. For the molecular spectrums you tend to use a classical model of electro-magnetic radiation - mainly because such theory exists and it is manageable. It would be far too complicated to apply a purely quantum mechanical description. A good text on molecular absorption discusses theory that works on a fairly small scale but it is unlikely to use the word "photon" or apply the theory down to that level.
If we go back to the comments made earlier by @Bored chemist , it may now start to make more sense to you.
Boredchemist has correctly identified that microwave radiation is way off the usual frequencies you observe for atomic absorption spectrums. It's even a bit low for the usual infra-red molecular absorption spectrums. However there is a part of those molecular movements that can happen at microwave frequencies - these tend to be complete rotations of the molecule rather than just agitation or vibration within the molecule. This phenomena is especially noticeable for water molecules and is one of the things that make microwave ovens work.
I have always assumed a monochromatic light has photon of a definite frequency w/o uncertainty principle involved.Quite often you are meant to make an assumption like that. As @alancalverd has just implied, you need to determine some level of idealisation and simplification which you are prepared to accept.
Best Wishes.
Post by: theThinker on 04/04/2023 00:28:33
Hi again,Thanks. I do have some sense of what you are saying, but the level of physics is beyond me. I only know the basic of Bohr's theory applied to the hydrogen atom where I can assume the photon is at work. Then, of course the changes in chemical energy in molecules formation or dissociation would definitely be way more complicated.But this is what most elementary textbook would assume.Which textbook? It would help to see what they said, or at least which topic they are covering and at what sort of level of study. Many school level textbooks would only mention a "photon" for the situation of raising an electron from one orbit to another (this will later be described as an atomic absorption). Meanwhile, a textbook for biochemists wishing to identify molecules by their infra-red absorption spectrum is referring to something different - in that situation you aren't really looking at "atomic absorption spectrums" but "molecular absorption spectrums". For the first (atomic absoption spectrums) you have electrons being directly moved to higher energy orbits. For the second (molecular absorption spectrums) you have much larger structures - molecules - and they are being made to vibrate or rotate. For the molecular spectrums you tend to use a classical model of electro-magnetic radiation - mainly because such theory exists and it is manageable. It would be far too complicated to apply a purely quantum mechanical description. A good text on molecular absorption discusses theory that works on a fairly small scale but it is unlikely to use the word "photon" or apply the theory down to that level.
If we go back to the comments made earlier by @Bored chemist , it may now start to make more sense to you.
Boredchemist has correctly identified that microwave radiation is way off the usual frequencies you observe for atomic absorption spectrums. It's even a bit low for the usual infra-red molecular absorption spectrums. However there is a part of those molecular movements that can happen at microwave frequencies - these tend to be complete rotations of the molecule rather than just agitation or vibration within the molecule. This phenomena is especially noticeable for water molecules and is one of the things that make microwave ovens work.I have always assumed a monochromatic light has photon of a definite frequency w/o uncertainty principle involved.Quite often you are meant to make an assumption like that. As @alancalverd has just implied, you need to determine some level of idealisation and simplification which you are prepared to accept.
Best Wishes.
So I think the discussion till now is sufficient for me.
Post by: Eternal Student on 04/04/2023 02:12:15
but the level of physics is beyond me.Not a problem. I must apologise for my comments, I do sometimes jump in too deep and may have sidetracked the forum thread.
The first set of questions you asked may have been adequately discussed by several replies and different people already. The second set of questions hasn't been discussed much. Here they are again:
Say we have a piece of copper isolated in a vacuum container. We now heat the temperature of the conatiner to above that of the copper. A net radiation photons would be absorbed by the copper raising its temperature.
1) How is the photon absorbed by the copper (ignore re-emission)
2) What actually happen when we say the temperature of the copper has risen.
Here are some brief answers:
1) Much the same as previously discussed for the more arbitrary atom(s) and molecules.
2) "What is temperature" has been a topic recently discussed here on this forum. It went on for many pages and lasted many weeks. So it's complicated or as complicated as you want it to be.
As a very simple starting point - it is a measure of the average kinetic energy of the particles in a substance. To say it even more simply, temperature is a measure of how much the particles are moving or "jiggling" about.
This is a collection of gas particles at medium temperature:
(https://upload.wikimedia.org/wikipedia/commons/6/6d/Translational_motion.gif?20080328033120)
Image taken from Wikipedia: https://upload.wikimedia.org/wikipedia/commons/6/6d/Translational_motion.gif?20080328033120
If you double the temperature of the gas then you should double the speed at which those particles move. If you half the temperature then you should half the speed at which they move.
Best Wishes.
Post by: alancalverd on 04/04/2023 09:42:38
If you double the temperature of the gas then you should double the speed at which those particles movePedant speaking: k.e. = ½ mv², so wouldn't the mean particle velocity increase by √2 ?
And whilst I'm wearing the pedant hat, the photon model ascribes a truly monochromatic spectrum to a single free photon. The perceived frequency does indeed depend on gravitational potential, relativistic shift, doppler, currency fluctuations and any other effect you want to mention, but as we see from Mossbauer experiments (and the Pound-Rebka experiment is satisfyingly definitive) you get the right answer by convoluting them with the theoretical single frequency.
Post by: Eternal Student on 04/04/2023 13:36:05
Pedant speaking: k.e. = ½ mv², so wouldn't the mean particle velocity increase by √2 ?Yes. Sorry. Too much rush to say something that was simple to understand on my part.
And whilst I'm wearing the pedant hat, the photon model ascribes a truly monochromatic spectrum to a single free photon.No or yes. It is an idealisation you can make. Just as you can construct QM solutions of a particle in a square potential well and even an infinite square well, even though a perfectly square (vertical sided) potential is unlikely to be a physical reality you could have.
The Heisenberg uncertainty principle in the form ΔE Δt ≥ ħ/2 is unavoidable in reality but you can choose to make the idealisation that the exact energy of a photon was known. Exactly how you measured the energy of the photon does not need to be part of the problem you are focusing on. Here's one quick guide to applying the principle to the emission of a photon by an atom: Example 7.3.3 on atomic transitions of this article https://phys.libretexts.org/Bookshelves/University_Physics/Book%3A_University_Physics_(OpenStax)/University_Physics_III_-_Optics_and_Modern_Physics_(OpenStax)/07%3A_Quantum_Mechanics/7.03%3A_The_Heisenberg_Uncertainty_Principle . We would be wanting to consider absorption rather than emission but it's quite similar.
To paraphrase the thing, even though school level physics tells you that an atom will absorb or release a photon of exactly one frequency (equal to the difference in energy levels etc.), it doesn't. There's a probability distribution for releasing or absorbing a photon of frequency ν which is something like a Gaussian distribution with mean = ν0 = the difference in energy levels between the electron orbitals and variance = ΔE/h with ΔE determined by the uncertainty relation above.
I'm at risk of sidetracking the thread again and/or confusing the OP, so we'll stop there.
Best Wishes.
Post by: alancalverd on 04/04/2023 14:01:12
Quote
Each time an excited state decays, the emitted energy is slightly different and, therefore, the emission line is characterized by a distribution of spectral frequencies (or wavelengths) of the emitted photons. As a result, all spectral lines are characterized by spectral widths. The average energy of the emitted photon corresponds to the theoretical energy of the excited state and gives the spectral location of the peak of the emission line. Short-lived states have broad spectral widths and long-lived states have narrow spectral widths.
The phrase I have italicised does not make sense, either by itself or in context! It should read "....emitted photons...", not singular, and therein lies the confusion. Remember that "photon" is a mathematical model of a phenomenon, and each photon in that model has a unique energy even if the ensemble is a broad continuum. Somewhere in my murky history I recall calculating the duration of photon absorption events if only to pour scorn on the fools who think instantaneous dose rate has something to do with radiation safety, and Δt is certainly not zero, but for a "particle" travelling at c, its only proper parameter is E, of which it is supremely certain.
Post by: Eternal Student on 05/04/2023 13:48:34
Ummm... Well I would have agreed to only half of what you said and I would have chosen to interpret what a photon is slightly differently to the way you have done. This post seems to have grown, sorry. It can be skipped and is not essential information for the OP ( @theThinker ). I'll put it under a spoiler to avoid cluttering the thread.
Spoiler: show
Best Wishes.
Post by: alancalverd on 05/04/2023 16:15:15
You seem to have ignored my point that the textbook you quote would make perfect sense if it talked about the average energy of a bunch of photons, but not the average energy of a single photon. What is my average age? On average, how many children do I have? One entity, one value: one village, a meaningful average and spread function.
Post by: Eternal Student on 05/04/2023 20:44:43
You seem to have ignored my point that the textbook you quote would make perfect sense if it talked about the average energy of a bunch of photons, but not the average energy of a single photon. What is my average age? On average, how many children do I have? One entity, one value: one village, a meaningful average and spread function.I do appreciate what you said, it was considered and thought about.
However, it was partially disagreed with. You are assuming you have an age - but if you were a Quantum mechanical object and not a macroscopic object, then you don't. However, you do have a wave function with which we can talk about and possibly even calculate the expected value of your age. You do not need a villlage of people, just one quantum person.
Best Wishes.
Post by: alancalverd on 06/04/2023 11:34:34
You are assuming you have an age - but if you were a Quantum mechanical object and not a macroscopic object, then you don't.That is "proof by assertion" - not permissible!
Post by: Bored chemist on 06/04/2023 12:03:12
How old is this electron?You are assuming you have an age - but if you were a Quantum mechanical object and not a macroscopic object, then you don't.That is "proof by assertion" - not permissible!
( )
Post by: alancalverd on 06/04/2023 17:27:30
Post by: Bored chemist on 06/04/2023 17:40:00
Most probably about 13.8 billion years, unless it has just escaped from a nucleus.
I think I recently posted about painting electrons purple; you can't do it.
Next question;
How old is this photon?
( )
Post by: alancalverd on 06/04/2023 21:04:19