Why doesn't the atom absorb heat energy when it is low?

[thaiqi]

(In my last thread)
Mentor Dale said:
"
An atom in the ground state can absorb energy from the environment including thermal radiation.
Once it has done so it will be excited and will no longer be in the ground state. An excited atom
can radiate and go to a lower energy state, but an atom in the ground state cannot radiate since
there is no lower energy state.
"
I then asked:
"
But how if the energy it absorbed is lower than that of the gap between the first excited state and ground state?
"
Dale said:
"
Then it is transparent to that radiation and cannot absorb it.
"
Now I have my question: Why cannot it absorb it ?

 

[Dale]

Just to confirm, I did recommend asking this question in the public forums

 

[thaiqi]

Sorry I am new to this forum. Did you mean public forum as another sub-forum different from General Physics? If so, please move it.

 

[Nugatory]

Now I have my question: Why cannot it absorb it ?

Because if it absorbed it, then its total energy would be neither the ground state energy nor the energy of the lowest excited state but somewhere in between, and that's physically impossible.
If you are trying to ask why the atom can only have those energy levels and not something in between, we'll need some quantum mechanics and even the most minimum answer is going to go beyond a B-level thread: google for "Schrodinger hydrogen atom" to get started.

But note that we're talking about radiation here. In your thread title you asked about heat energy, and it never makes sense to talk about a single atom absorbing heat energy. Heat energy is (loosely speaking) the kinetic energy of a large number of molecules moving at random.

 

[Dale]

Sorry I am new to this forum. Did you mean public forum as another sub-forum different from General Physics? If so, please move it.

No worries, you did correctly!

 

[thaiqi]

No worries, you did correctly!

Thanks.

 

[thaiqi]

Because if it absorbed it, then its total energy would be neither the ground state energy nor the energy of the lowest excited state but somewhere in between, and that's physically impossible.
If you are trying to ask why the atom can only have those energy levels and not something in between, we'll need some quantum mechanics and even the most minimum answer is going to go beyond a B-level thread: google for "Schrodinger hydrogen atom" to get started.

But note that we're talking about radiation here. In your thread title you asked about heat energy, and it never makes sense to talk about a single atom absorbing heat energy. Heat energy is (loosely speaking) the kinetic energy of a large number of molecules moving at random.

Thanks. You are more correct about the title should be "thermal radiation energy". By my question, I mean why I can't regard that the atom absorbs the energy and at the same period radiates out the same amount?

 

[Dale]

Why cannot it absorb it ?

When you solve the Schrodinger equation for an atom, one of the things that naturally falls out of the solution is that only discrete energy levels are possible. If an atom is exposed to radiation for which it has a corresponding energy level then it can absorb it and transfer to an excited state. Conversely, if no corresponding state exists then it cannot transfer and therefore cannot absorb the radiation. This is the cause of the absorption lines in gas spectra.

 

[thaiqi]

When you solve the Schrodinger equation for an atom, one of the things that naturally falls out of the solution is that only discrete energy levels are possible. If an atom is exposed to radiation for which it has a corresponding energy level then it can absorb it and transfer to an excited state. Conversely, if no corresponding state exists then it cannot transfer and therefore cannot absorb the radiation. This is the cause of the absorption lines in gas spectra.

Can I regard that the atom absorbs the energy and at the same period radiates out the same amount?

 

[anorlunda]

Can I regard that the atom absorbs the energy and at the same period radiates out the same amount?

Are you trying to ask about scattering of light?

 

[thaiqi]

Are you trying to ask about scattering of light?

Here I didn't relate it to the scattering of light. Why do you think there is connection between them?

 

[Dale]

Can I regard that the atom absorbs the energy and at the same period radiates out the same amount?

No. The atom cannot absorb the energy. There is nowhere in the atom for it to go. Such a scheme would therefore not conserve energy.

 

[thaiqi]

Do you mean you regard that if it absorbs and radiates in the same period, the energy it absorbs and radiates out cannot balance ?

 

[Dale]

Do you mean you regard that if it absorbs and radiates in the same period, the energy it absorbs and radiates out cannot balance ?

If it absorbs energy then that means that the energy enters into the atom. There is nowhere for the energy to go, so it cannot be absorbed.

It sounds like you are trying to redefine the word “absorb” such that it is possible for an atom to “absorb” energy without that energy entering the atom. This is not what “absorb” means. When an atom absorbs energy its energy level increases.

 

[thaiqi]

I guess it absorbs under the thermal agitation in its environment, and radiates out as classical electrodynamics said when it is accelerating. The net result is zero. You could say the net absorption is zero.

 

[Dale]

I guess it absorbs under the thermal agitation in its environment

Only if the thermal agitation has an energy corresponding to an available transition. If not, then it cannot absorb the energy.

At this point it seems like you have an agenda you wish to push and are not here to learn. Please review the forum rules which prohibit personal speculation.

 

[thaiqi]

So you regard that contrary to classical electrodynamics, even it is accelerating, it doesn't radiate electromagnetic wave ?

 

[Dale]

So you regard that contrary to classical electrodynamics, even it is accelerating, it doesn't radiate electromagnetic wave ?

The atom is neutral. According to classical electrodynamics it won’t radiate because it accelerates.

 

[sophiecentaur]

When you solve the Schrodinger equation for an atom, one of the things that naturally falls out of the solution is that only discrete energy levels are possible.

It's worth pointing out that the familiar Hydrogen Atom equation (one proton and one electron) shows the first Energy Level above ground state to be around 10eV (iirc) which is an UV transition. Thermal frequencies are associated with molecular transitions so the H atom model is not the best to hold in your mind when discussing this. The states associated with molecular vibration are more what you want but the model is not so familiar. The same principle applies, of course.

 

[Dale]

It's worth pointing out that the familiar Hydrogen Atom equation (one proton and one electron) shows the first Energy Level above ground state to be around 10eV (iirc) which is an UV transition. Thermal frequencies are associated with molecular transitions so the H atom model is not the best to hold in your mind when discussing this

Agreed, which is also why many gasses are transparent to IR or visible light.

 

[thaiqi]

The atom is neutral. According to classical electrodynamics it won’t radiate because it accelerates.

Well, I meant it radiates through the electron outside of the nucleus.

 

[Dale]

Well, I meant it radiates through the electron outside of the nucleus.

The electrons and the nucleus accelerate together

 

[thaiqi]

The electrons and the nucleus accelerate together

I mean that the electron rotates around the nucleus.

 

[Dale]

I mean that the electron rotates around the nucleus.

The observation that this radiation does not happen is one of the observations that falsifies classical electromagnetism

 

[sophiecentaur]

I mean that the electron rotates around the nucleus.

That's a very old model by Neils Bohr - pre Quantum Theory, iirc. Read about it on Wiki.

 

[thaiqi]

The observation that this radiation does not happen is one of the observations that falsifies classical electromagnetism

So it is contrary to what classical electrodynamics said?

 

[sophiecentaur]

So it is contrary to what classical electrodynamics said?

Classical theory does not include the Quantisation of Energy and it would allow for all wavelengths to interact with all charge systems.

 

[Dale]

So it is contrary to what classical electrodynamics said?

Yes. The classical electrodynamics calculations give a lifetime for the hydrogen atom on the order of 10^-11 s. Since the hydrogen ground state is stable this is contrary to classical electrodynamics

 

[Mister T]

By my question, I mean why I can't regard that the atom absorbs the energy and at the same period radiates out the same amount?

Can you think of an experiment that would distinguish between the absorption and re-emission versus no absorption at all? If you can't, then you are talking about a distinction without a difference. You are just arguing semantics, that is, the definition of what it means to absorb.

When an atom absorbs a photon, its energy level goes up by an amount equal to the energy of the photon. There are experiments that measure this increase in energy. In every case, the increase equals the difference in energy levels of two states of the atom. If the photon energy doesn't match this energy difference, then the atom's energy level doesn't change. We describe that by saying that the photon is not absorbed. If you want to insist that we are not using the same meaning of absorb as you are, then all I can tell you is that the meaning we are using matches the meaning used in the textbooks and in the literature. The meaning that you are using doesn't, and I hope you can appreciate that the rest of the world will not change its meaning to match yours.

 

[sophiecentaur]

Can you think of an experiment that would distinguish between the absorption and re-emission versus no absorption at all?

I can. Absorption and re-emission introduces random phase delays where straight scattering will not randomise the phase. There are many experiments that could show a difference in coherence of what comes out, according to the mechanism.

 

[thaiqi]

Thanks to everyone for your opinion.

 

[DanMP]

Absorption and re-emission introduces random phase delays

Why? Please elaborate.Huygens principle states that "each point on wavefront act as a fresh source of distribution of light". Are those "fresh sources" introducing random phase delays?

 

[sophiecentaur]

Why? Please elaborate.Huygens principle states that "each point on wavefront act as a fresh source of distribution of light". Are those "fresh sources" introducing random phase delays?

When an atom goes into an excited state, there is a random element in the time it takes to re-emit its photon so the coherence is destroyed. The only time this effect doesn't happen is in a laser, when there is stimulated emission. But that only happens (to any significant degree) when you have a Population Inversion. It's a long time since I learned this but it only happens under the 'right conditions, only in some molecules and when vast numbers of the atoms are in the excited state.
So this doesn't constitute Huygens sources, which is when the whole wave interacts with the bulk of the substance, rather than with individual atoms. These Huygens sources are not 'real particles' but a mathematical construct, invented before Calculus was available for diffraction calculations and are much the same thing (afaiaa).

 

[sophiecentaur]

If it worked by absorption and re-emission, there would be no Rectilinear Propagation, except in a vacuum.

 

[DanMP]

When an atom goes into an excited state, there is a random element in the time it takes to re-emit its photon so the coherence is destroyed.

True, but what if the atom/molecule goes into a virtual state (more probable than into an excited state, when the material is transparent ...)? In this case it is not a real absorption but a failed one, always and promptly followed by the re-emission ... It would be almost like with the Huygens sources ...

If it worked by absorption and re-emission, there would be no Rectilinear Propagation, except in a vacuum.

You are forgetting the Fresnel part in Huygens–Fresnel principle ...