<p>You cannot have free protons without electrons. Plasmas, in general, are electrically neutral.</p>
<p>It is usually electrons that dominate the scattering (note that a point-like charge cannot absorb a photon and conserve energy and momentum) in a plasma at low photon energies. That is basically due to their much lower masses (classically you can think of the electric field of the incoming light accelerating the particles). NB. Electrons in the electric field of ions can emit or absorb photons in a process called (inverse) bremsstrahlung.</p>
<p>High energy (&gt;MeV) photons are capable of causing transitions in the energy levels of nuclei, so can be absorbed. At even higher energies the photons can interact with a nucleus and create particle/anti-particle pairs - pair production.</p>
<p>Very high energy (&gt;100 MeV) photons can scatter from protons and neutrons because these particles have internal quark structure. It is even possible for photons to be absorbed, creating short-lived 3 quark resonances.</p>

CC BY-SA 4.0
2020-07-24T22:14:18.990Z
If protons and nuclei do not generally absorb and/or emit most photons, how do we know which elements are in which stars?  The emissions from all the atoms in a star, from hydrogen and helium to the 'metals', are from completely naked, fully ionized nuclei, correct?
Kurt Hikes
CC BY-SA 4.0
2020-07-24T23:01:50.827Z
@kurthikes Spectral absorption and emission lines are due to the transitions of bound electrons in neutral or *partially* ionised atoms.
ProfRob
CC BY-SA 4.0
2020-07-24T23:10:12.597Z
@kurthikes The photosphere of the Sun is not completely ionised.
ProfRob
CC BY-SA 4.0
2020-07-25T00:28:47.257Z
Free neutrons (also mentioned in the question) aren't very abundant because they are unstable - they decay into other particles with a half-life of about 10 minutes.
Eric Jensen
CC BY-SA 4.0
2020-07-24T21:05:17.813Z
2020-07-24T23:07:53.573Z
2020-07-24T23:07:53.573Z
ProfRob
<p>In a plasma, or wherever, do the completely ionized nuclei commonly absorb much EM radiation?  Or any free neutrons or protons?  Can astronomers detect this?  Enough so that astronomers take it into account in their observations?</p>
<p>I've read elsewhere that protons, neutrons and nuclei, even when free of any electrons, usually (or only, depending on who you read) aborb and/or emit only hard X-rays and gamma rays...</p>
<p>Is this true?
.
.
.
.
.
P.S.:  So, free electrons cannot absorb or emit photons by themselves, even when accelerating or decelerating rapidly?</p>
<p>What about an atom or molecule?  They won't absorb/emit photons, whether they are neutral OR ionized, even if accelerating/decelerating rapidly?  Unless their electrons change energy levels?</p>
<p>When electrons in a cathode ray tube hit the screen, is it only the phosphor molecule in the screen that emits any kind of photon?  And the valence electron(s) in the phosphor jump quantum energy levels (n or sublevels l) to emit; it's not from the shaking due to just being hit?</p>
<p>Sorry for all of the sudden new questions, but I am still confused to this day....</p>
<p>P.P.S.:  Don't neutral atoms absorb and emit infrared all the time when jittering back and forth?  Don't everyday objects, especially white ones, absorb most visible light, the re-emit infrared?  It's not just scattering, is it?...</p>
<p>And inelastic Compton scattering must involve actual absorption and re-emission, mustn't it?  Being inelastic and all...</p>
<p>I apologize again for the outburst of questions....</p>

CC BY-SA 4.0
2020-07-24T21:41:39.243Z
Here's an answer on Physics.SE (also from Rob Jeffries) that explains why a free electron cannot absorb a photon: https://physics.stackexchange.com/a/225538/123208
PM 2Ring
CC BY-SA 4.0
2020-07-24T22:10:14.917Z
Free electrons DO absorb and emit photons!  It is called Thomson and/or Compton scattering....
Kurt Hikes
CC BY-SA 4.0
2020-07-24T22:15:27.940Z
Also, if free electrons didn't emit light, why does the sun shine? It is made of mostly free protons and electrons...
Kurt Hikes
CC BY-SA 4.0
2020-07-24T23:06:01.057Z
Scattering is not absorption. "Free" electrons that are accelerating in the electric field of a positive nucleus can result in the emission or absorption of light. It's called bremsstrahlung.
ProfRob
CC BY-SA 4.0
2021-08-06T00:04:49.867Z
Do you feel the current answer is sufficient? I've been going through my old questions and [looking for answers I'd forgotten to accept](https://astronomy.meta.stackexchange.com/a/776/7982) and I ran across this post by accident.
uhoh
CC BY-SA 4.0
2022-01-29T03:15:33.947Z
What free neutrons?
PM 2Ring
<observational-astronomy><spectroscopy><plasma-physics><electromagnetic-spectrum><neutrons>
Do free protons and neutrons absorb much radiation? To affect astronomers' observations? If so, at what wavelength(s)?
Kurt Hikes
# Do free protons and neutrons absorb much radiation? To affect astronomers' observations? If so, at what wavelength(s)?

(Asked by: username_0 on 2020-07-24)

In a plasma, or wherever, do the completely ionized nuclei commonly absorb much EM radiation? Or any free neutrons or protons? Can astronomers detect this? Enough so that astronomers take it into account in their observations?

I've read elsewhere that protons, neutrons and nuclei, even when free of any electrons, usually (or only, depending on who you read) aborb and/or emit only hard X-rays and gamma rays...

Is this true?
.
.
.
.
.
P.S.: So, free electrons cannot absorb or emit photons by themselves, even when accelerating or decelerating rapidly?

What about an atom or molecule? They won't absorb/emit photons, whether they are neutral OR ionized, even if accelerating/decelerating rapidly? Unless their electrons change energy levels?

When electrons in a cathode ray tube hit the screen, is it only the phosphor molecule in the screen that emits any kind of photon? And the valence electron(s) in the phosphor jump quantum energy levels (n or sublevels l) to emit; it's not from the shaking due to just being hit?

Sorry for all of the sudden new questions, but I am still confused to this day....

P.P.S.: Don't neutral atoms absorb and emit infrared all the time when jittering back and forth? Don't everyday objects, especially white ones, absorb most visible light, the re-emit infrared? It's not just scattering, is it?...

And inelastic Compton scattering must involve actual absorption and re-emission, mustn't it? Being inelastic and all...

I apologize again for the outburst of questions....

**Tags:** observational-astronomy, spectroscopy, plasma-physics, electromagnetic-spectrum, neutrons

> **Comments:**
> **username_1**: Here's an answer on Physics.SE (also from Rob Jeffries) that explains why a free electron cannot absorb a photon: https://physics.stackexchange.com/a/225538/123208  
> **username_0**: Free electrons DO absorb and emit photons!  It is called Thomson and/or Compton scattering....  
> **username_0**: Also, if free electrons didn't emit light, why does the sun shine? It is made of mostly free protons and electrons...  
> **username_2**: Scattering is not absorption. "Free" electrons that are accelerating in the electric field of a positive nucleus can result in the emission or absorption of light. It's called bremsstrahlung.  
> **username_3**: Do you feel the current answer is sufficient? I've been going through my old questions and [looking for answers I'd forgotten to accept](https://astronomy.meta.stackexchange.com/a/776/7982) and I ran across this post by accident.  
> **username_1**: What free neutrons?  

---

## 1 answers

### Answer by username_2 on 2020-07-24. Score: 6

You cannot have free protons without electrons. Plasmas, in general, are electrically neutral.

It is usually electrons that dominate the scattering (note that a point-like charge cannot absorb a photon and conserve energy and momentum) in a plasma at low photon energies. That is basically due to their much lower masses (classically you can think of the electric field of the incoming light accelerating the particles). NB. Electrons in the electric field of ions can emit or absorb photons in a process called (inverse) bremsstrahlung.

High energy (>MeV) photons are capable of causing transitions in the energy levels of nuclei, so can be absorbed. At even higher energies the photons can interact with a nucleus and create particle/anti-particle pairs - pair production.

Very high energy (>100 MeV) photons can scatter from protons and neutrons because these particles have internal quark structure. It is even possible for photons to be absorbed, creating short-lived 3 quark resonances.

> **Comments:**
> **username_0**: If protons and nuclei do not generally absorb and/or emit most photons, how do we know which elements are in which stars?  The emissions from all the atoms in a star, from hydrogen and helium to the 'metals', are from completely naked, fully ionized nuclei, correct?  
> **username_2**: @username_0 Spectral absorption and emission lines are due to the transitions of bound electrons in neutral or *partially* ionised atoms.  
> **username_2**: @username_0 The photosphere of the Sun is not completely ionised.  
> **username_4**: Free neutrons (also mentioned in the question) aren't very abundant because they are unstable - they decay into other particles with a half-life of about 10 minutes.  

---

