Can an atom absorb a photon, yet its total kinetic energy is decreased?

[avicenna]

Let's assume an atom consists of the nucleus and electrons as point particles. Take the inertial frame to be that of the fixed laboratory. Its total energy consists of the total kinetic and potential energy of the system of particles.If an electron absorbs a photon of energy E, the total energy of the atom increases by E. Can it happen that this increase in energy result in an increase of its total potential energy = E + Del(potential energy) and its total kinetic energy decreases by Del(potential energy).

 

[DrClaude]

First, it is not the electron that absorbs the photon, it is the system nucleus + electron.

Otherwise, I am not sure what you are after. If you take the atom as a whole, then yes its kinetic energy can decrease. As the photon carries momentum, if the atom was moving initially in the direction opposite to the photon, then the kinetic energy of the atom will decrease.

If you are taking about the internal states, then the answer is no. The viral theorem tells you that for the Coulomb potential, you have
$$
2 \braket{T} = - \braket{V}
$$
so the magnitude of kinetic energy and the potential energy must vary in lockstep.

 

[avicenna]

"First, it is not the electron that absorbs the photon, it is the system nucleus + electron."

But the wiki seems to state otherwise:
(Bohr's theory)
"Electrons can only gain and lose energy by jumping from one allowed orbit to another, absorbing or emitting electromagnetic radiation with a frequency determined by the energy difference of the levels according to the Planck relation:

 

[Andy Resnick]

Let's assume an atom consists of the nucleus and electrons as point particles. Take the inertial frame to be that of the fixed laboratory. Its total energy consists of the total kinetic and potential energy of the system of particles.If an electron absorbs a photon of energy E, the total energy of the atom increases by E. Can it happen that this increase in energy result in an increase of its total potential energy = E + Del(potential energy) and its total kinetic energy decreases by Del(potential energy).

If I understand your question, the answer is 'yes', it's the basic principle of laser cooling- cooling atoms via laser light.

 

[PeroK]

"First, it is not the electron that absorbs the photon, it is the system nucleus + electron."

But the wiki seems to state otherwise:
(Bohr's theory)
"Electrons can only gain and lose energy by jumping from one allowed orbit to another, absorbing or emitting electromagnetic radiation with a frequency determined by the energy difference of the levels according to the Planck relation:

Spectroscopy applies to atoms, not to electrons. It's very much the atom that is absorbing and emitting radiation in discrete wavelengths.

 

[berkeman]

But the wiki seems to state otherwise:
(Bohr's theory)
"Electrons can only gain and lose energy by jumping from one allowed orbit to another

Orbit around what?

 

[avicenna]

Orbit around what?

But it is what the wiki says and how the textbook explains Bohr's theory, a classical theory. If you insist we should not invoke Bohr's theory, then I have no comment.

 

[berkeman]

But it is what the wiki says and how the textbook explains Bohr's theory, a classical theory. If you insist we should not invoke Bohr's theory, then I have no comment.

Say what? You seem to be misinterpreting the responses you've received in this thread, and misunderstanding the wiki article. The energy of the photon is absorbed by the system of the electron and the nucleus that it is bound to. That *system* is what can absorb that photon energy (given the right circumstances). It is not a free electron that is absorbing the energy.

Have any comments on that?

 

[avicenna]

Say what? You seem to be misinterpreting the responses you've received in this thread, and misunderstanding the wiki article. The energy of the photon is absorbed by the system of the electron and the nucleus that it is bound to. That *system* is what can absorb that photon energy (given the right circumstances). It is not a free electron that is absorbing the energy.

Have any comments on that?

Bohr's theory is clear about how an electron of an atom (nothing about free electrons) absorbs a photon; the electron goes to a higher energy state.

 

[Ibix]

Bohr's theory is clear about how an electron of an atom (nothing about free electrons) absorbs a photon; the electron goes to a higher energy state.

But the energy states aren't something the electron has on its own. In the Bohr model, the energy states correspond to different orbits around the nucleus. So it's the atom that absorbs the photon; the electron doesn't have "higher energy states" without being part of an atom.

 

[avicenna]

But the energy states aren't something the electron has on its own. In the Bohr model, the energy states correspond to different orbits around the nucleus. So it's the atom that absorbs the photon; the electron doesn't have "higher energy states" without being part of an atom.

The electron's total energy (kinetic + potential ) increases by E = h f

 

[Ibix]

The electron's total energy (kinetic + potential ) increases by E = h f

If so, the nucleus doesn't change its state of motion because there's no energy to spare. Right?

Or it's possible that you are considering a model that is a bit too idealised (assuming the nucleus is effectively an infinite mass) to answer the question you are asking.

 

[PeroK]

The electron's total energy (kinetic + potential ) increases by E = h f

What about conservation of momentum? When an atom emits a photon, it's the atom that recoils; not the electron.

 

[berkeman]

The electron's total energy (kinetic + potential ) increases by E = h f

Here, this should clear things up for you a bit...

https://en.wikipedia.org/wiki/Electron_configuration

 

[avicenna]

If so, the nucleus doesn't change its state of motion because there's no energy to spare. Right?

Or it's possible that you are considering a model that is a bit too idealised (assuming the nucleus is effectively an infinite mass) to answer the question you are asking.

The energy absorbed by the electron is E=hf; it is the increase in the total energy of the electron relative to the mass-center (kinetic+potential).

 

[avicenna]

What about conservation of momentum? When an atom emits a photon, it's the atom that recoils; not the electron.

Correct.

 

[Ibix]

The energy absorbed by the electron is E=hf; it is the increase in the total energy of the electron relative to the mass-center (kinetic+potential).

So where do you think the energy to start the atom moving (or change its state of motion) comes from?

 

[Herman Trivilino]

"First, it is not the electron that absorbs the photon, it is the system nucleus + electron."

But the wiki seems to state otherwise:
(Bohr's theory)
"Electrons can only gain and lose energy by jumping from one allowed orbit to another, absorbing or emitting electromagnetic radiation with a frequency determined by the energy difference of the levels according to the Planck relation:

Bohr's theory is part of the old quantum theory and only works in special cases. Anyway, it's the atom, not the electron, that absorbs or emits radiation.

 

[sophiecentaur]

Bohr's theory is clear about how an electron of an atom (nothing about free electrons) absorbs a photon; the electron goes to a higher energy state.

I wouldn't say that the wording of the version of Bohr's theory that you remember is clear because that statement ignores the need for a photon to interact with the system. We have moved on from there to deal with many other forms of energy changes in quantum systems of charges.
There really is very little point in trying to resurrect early models and insist they must be correct.

 

[Dale]

The electron's total energy (kinetic + potential ) increases by E = h f

The potential energy belongs to the atom, not the electron. The electron doesn’t have potential energy in its own. The potential energy describes the relationship between the electron and the nucleus, which is the atom.

 

[PeterDonis]

the wiki

Is not a good source to be using, you should be looking at textbooks or peer-reviewed papers.

(Bohr's theory)

Is an outdated model. You want to be looking at a modern treatment. There are many good textbooks that cover this material.

 

[PeterDonis]

how the textbook

What textbook? The only specific source you've mentioned is Wikipedia.

explains Bohr's theory, a classical theory.

No, Bohr's theory is not a classical theory. It was an early version (now outdated, as I have said) of quantum theory.

If you insist we should not invoke Bohr's theory, then I have no comment.

Why would you want to use an outdated theory when there are good modern treatments of the same thing?

 

[avicenna]

The potential energy belongs to the atom, not the electron. The electron doesn’t have potential energy in its own. The potential energy describes the relationship between the electron and the nucleus, which is the atom.

You set me thinking. I can't yet give you a proper reply now - probably later.

I have some observations to make relating to the solar system. You would then
say the earth does not have potential energy on its own, only the solar system
has potential energy. But in the real world, we calculate the system of the
earth and the sun only and its "approximation" seems to be good enough.

In Bohr's theory, there was only one electron of the hydrogen atom, so ok. In
many-electron atoms, you expect me to include potentials between electrons. Are
you not asking me to solve a n-body problem, n > 2,3,4...No one has yet solved
the 3-body problem

 

[avicenna]

What textbook? The only specific source you've mentioned is Wikipedia.No, Bohr's theory is not a classical theory. It was an early version (now outdated, as I have said) of quantum theory.Why would you want to use an outdated theory when there are good modern treatments of the same thing?

I only know Bohr's theory. Tell me if my reliance on it misled me into wrong conclusions.

 

[PeterDonis]

I only know Bohr's theory. Tell me if my reliance on it misled me into wrong conclusions.

As far as I can tell, you aren't even using Bohr's theory correctly. More generally, you don't seem to understand how potential energy works.

You would then
say the earth does not have potential energy on its own, only the solar system
has potential energy. But in the real world, we calculate the system of the
earth and the sun only and its "approximation" seems to be good enough.

Sure, the Earth-Sun approximation works for many purposes. But you aren't even doing that. You're trying to make sense of the potential energy of just the Earth alone--that's what asking for the potential energy of the electron is analogous to.

In
many-electron atoms, you expect me to include potentials between electrons

Nobody is asking you to do that. Everyone keeps telling you that the potential energy, even for a one-electron atom, is not a property of the electron. It's a property of the atom. That is, the electron-nucleus system. Analogous to the Earth-Sun system.

 

[PeterDonis]

No one has yet solved
the 3-body problem

Here you're talking classical physics. Atoms are not classical objects. They are quantum objects. For quantum objects we don't even have an exact general solution for the two- or even one-body problem. Indeed, in quantum field theory, where the vacuum state itself is complicated, we don't even have an exact solution for the zero-body problem. But none of that prevents us from constructing models that make accurate predictions, even if one has to use successive approximations to do it. The question is what approximations to use.

 

[Dale]

You would then say the earth does not have potential energy on its own, only the solar system has potential energy.

Yes.

But in the real world, we calculate the system of the earth and the sun only and its "approximation" seems to be good enough

Sure. Although you do have to specify the exact purpose for which a given approximation is considered good enough.

you expect me to include potentials between electrons. Are
you not asking me to solve a n-body problem, n > 2,3,4...No one has yet solved

I am not expecting or asking either. I am simply telling you that the PE belongs to the atom, not the electron.

This is important because an electron has no internal structure so it cannot absorb a photon at all. It can only scatter the photon.

 

[sophiecentaur]

You would then
say the earth does not have potential energy on its own,

Potential energy is always relative to some frame. It takes two to do the PE tango.

 

[avicenna]

Potential energy is always relative to some frame. It takes two to do the PE tango.

It is just debating on semantics. An electron coming to a proton from infinity has PE = -K/r. Its total energy relative to proton (theory same for mass center ) T = PE + 1/2 mv², v is relative velocity to proton. The textbooks do identity "a potential energy of an electron".

 

[sophiecentaur]

It is just debating on semantics. An electron coming to a proton from infinity has PE = -K/r. Its total energy relative to proton (theory same for mass center ) T = PE + 1/2 mv², v is relative velocity to proton. The textbooks do identity "a potential energy of an electron".

Your problem here is a matter of degree. For two equal masses and charges, neither mass 'has PE'. So where's the difference when it's an electron and a proton?
That is not semantics.