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Photoelectric Effect


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It is not long since I was reading about photoelectric effect, and some interesting questions impressed upon myself, which I will be obliged if you answered in a simple manner:

1. Okay. Photons transfer energy to electrons and they, in turn are ejected following certain rules. But what happens to the photons which gave (all?) of their energy? Do they just lie dormant there?

2. Where I was reading, it was all about electrons. But can photons wrung out protons, neutrons, etc. too? If so, then are they capable of causing subatomic particles to dissect into their components too?

3. Last of all, are photons pure energy or kind of particle-energy entities?

Thanks.

 

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A photon is energy that electrons can absorb; thus, increasing the mass-energy of the electron. Later, that electron may loose mass-energy when it emits a photon.

 

Photons are sometimes emitted by particle decay, for example a neutron is radioactive when isolated from an atom, and may emit an electron and proton.

 

Photons are particle-waves that have no mass (i.e., only energy).

 

I am not an expert, but this is pretty close to accurate.

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2. Where I was reading, it was all about electrons. But can photons wrung out protons, neutrons, etc. too? If so, then are they capable of causing subatomic particles to dissect into their components too?

 

 

Nuclei can absorb photons as well, and emit a proton or neutron.

 

A photon is energy that electrons can absorb; thus, increasing the mass-energy of the electron. Later, that electron may loose mass-energy when it emits a photon.

No, the mass energy of the electron itself is not affected. The electron may have kinetic energy after being ejected, and the mass-energy of the atom will change, though.

 

The electron will not emit a photon due to any sort of decay; you can get photons emitted if you accelerate the electron.

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...The mass energy of the electron itself is not affected [when absorbing a photon]

Why not? An electron in a lower state in an atom absorbs an electron and goes to a higher state. The electron gets a less favourable electrostatic potential (say, it's farther from the nucleus) so its mass-energy must be higher. What would be wrong with that?

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Nuclei can absorb photons as well, and emit a proton or neutron.

 

 

No, the mass energy of the electron itself is not affected. The electron may have kinetic energy after being ejected, and the mass-energy of the atom will change, though.

 

The electron will not emit a photon due to any sort of decay; you can get photons emitted if you accelerate the electron.

I was thinking about the laser, which I think I learned that electrons absorb a photon, jump to a higher orbital, and release a photon when they decay to lower orbital.

Edited by EdEarl
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Photons are particle-waves that have no mass (i.e., only energy).

 

The photon is destroyed by the interaction where its energy is transferred. Photons have energy, energy is a property of stuff not stuff itself. Photons also have other properties.

 

I am getting confused. First of all, if photons do contain particles, then wh don't they have mass? Secondly, if they are particle-waves, then what happens to the particle part of the photon when it transfers its energy?

 

Nuclei can absorb photons as well, and emit a proton or neutron.

Thank you. That answers half of point 2. But can photons similarly dissect the subatomic components too? Or is that possible only by collision of atoms?

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I am getting confused. First of all, if photons do contain particles, then wh don't they have mass? Secondly, if they are particle-waves, then what happens to the particle part of the photon when it transfers its energy?

 

Photons don't contain other particles.

 

Particle/wave describes photon behavior,not composition. The other properties Klaynos mentioned would include momentum, angular momentum/polarization and electric and magnetic fields.

 

Thank you. That answers half of point 2. But can photons similarly dissect the subatomic components too? Or is that possible only by collision of atoms?

 

It's far more efficient to collide massive particles to do particle physics. I don't know for sure that it's impossible with photons, as long as there is an electromagnetic interaction to exploit, but creating TeV scale photons requires TeV scale charged particles, so we just use the particles. Much easier to do in many ways.

 

I was thinking about the laser, which I think I learned that electrons absorb a photon, jump to a higher orbital, and release a photon when they decay to lower orbital.

 

Two things about that: 1) the atom absorbs and emits the photon and 2) that's not the photoelectric effect, and the topic of discussion is the photoelectric effect. In the PEE the electron is ionized, so there can be no discussion of the electron emitting a photon. Free electrons do not do that.

 

Why not? An electron in a lower state in an atom absorbs an electron and goes to a higher state. The electron gets a less favourable electrostatic potential (say, it's farther from the nucleus) so its mass-energy must be higher. What would be wrong with that?

 

As above, that's not the photoelectric effect. One cannot assign the mass change to any individual particle in the atom when the atom's internal energy is changed. It's the mass of the whole system that's different.

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Two things about that: 1) the atom absorbs and emits the photon and 2) that's not the photoelectric effect, and the topic of discussion is the photoelectric effect. In the PEE the electron is ionized, so there can be no discussion of the electron emitting a photon. Free electrons do not do that.

True, I lost track of the subject of the post. ty

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True, I lost track of the subject of the post. ty

 

I understand the confusion, and the difficulties of statements that are true under some conditions but not necessarily in general. We just have to try and remember that context always matters.

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One cannot assign the mass change to any individual particle in the atom when the atom's internal energy is changed. It's the mass of the whole system that's different.

That would surprise me. Since electrons are much lighter than the nucleus, we can and do observe the effect of the change in relativistic mass of the electrons. The mass change introduces a correction in the orbital energy, which is well measured sice it depends on each orbital and has different effects on the wavelengths of each transitions - and we can measure them with astonishing precision.

 

At least for the hydrogen atom, accurate solutions are known, and they fit the measures to many digits - but only half as many digits if the electron's relativistic mass change is neglected. And as expected from the energy levels, this change depends on the orbital.

 

So it looks logical to me that the electron's relativistic mass changes with the orbital, that we can attribute the kinetic energy change to one particle mainly (the nucleus gets a bit as well, in a proportion easy to compute).

 

For the electrostatic part of the absorbed energy, a distinction between electron and nucleus would make little sense, since it's an interaction.

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Enthalpy, on 03 Jun 2013 - 12:58, said:

That would surprise me. Since electrons are much lighter than the nucleus, we can and do observe the effect of the change in relativistic mass of the electrons.

Citation, please.

 

Enthalpy, on 03 Jun 2013 - 12:58, said:

The mass change introduces a correction in the orbital energy, which is well measured sice it depends on each orbital and has different effects on the wavelengths of each transitions - and we can measure them with astonishing precision.

I think you'll find the corrections are for the energy of the electron. Not the rest mass.

Enthalpy, on 03 Jun 2013 - 12:58, said:

At least for the hydrogen atom, accurate solutions are known, and they fit the measures to many digits - but only half as many digits if the electron's relativistic mass change is neglected. And as expected from the energy levels, this change depends on the orbital.

Aye, there's the rub. Relativistic mass is merely a proxy for the energy. Mass ≠ relativistic mass. Relativistic mass is a zombie concept. It will not die, and feeds on brains.

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Man, the importance of relativistic mass on orbital energy is in every course about orbitals...

 

The question was not about relativistic versus rest mass, but whether the photon's energy absorbed by the atom can be attributed to the electron. And for the kinetic part of this energy, the answer is a clear "yes, essentially to the electron", because essentially the electron moves, the nucleus far less so.

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The question was not about relativistic versus rest mass, but whether the photon's energy absorbed by the atom can be attributed to the electron.

 

No, the question was whether the electron absorbed the energy, not whether it received the bulk of the KE (and besides, "bulk of" ≠ "all"). Electrons cannot absorb photons, because such an interaction would not allow both momentum and energy to be conserved. i.e. you cannot ignore the fact that the electron is attached to an atom, and the system absorbs the photon. Saying that the electron absorbs the photon is a lazy description that can give rise to misconceptions.

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