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When incident light passes through a hydrogen gas, for example, does it have 50% chance (since it's a 1:1 ratio of protons to electrons) of getting absorbed by the proton? Any chance at all? If no, why not? Does a proton have a "bandgap"? If yes, what happens when a photon is absorbed by a proton?

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    $\begingroup$ Why would you think that the chance is 50%? Also, where would the energy go? The electron can go into higher orbitals, what is the proton supposed to do? $\endgroup$
    – ACuriousMind
    Nov 10, 2015 at 22:15
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    $\begingroup$ @ACuriousMind that's exactly what I'm curious about... $\endgroup$
    – Sparkler
    Nov 10, 2015 at 22:16
  • $\begingroup$ Like many (most? all?) particle interactions, it is going to depend on the energy of the two. For instance, the $\gamma+p$ interaction for a CMB photon and an ultra-high energy cosmic ray proton leads to pion production, but that's not going to happen for less-energetic $p$'s. $\endgroup$
    – Kyle Kanos
    Nov 10, 2015 at 22:18
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    $\begingroup$ Are you aware of cross-section when it comes to particle physics? $\endgroup$
    – Kyle Kanos
    Nov 10, 2015 at 22:40
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    $\begingroup$ @ACuriousMind What about nucleon excited states? It is a composite particle, after all. $\endgroup$
    – Declan
    Nov 10, 2015 at 22:50

4 Answers 4

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A system can absorb a photon if the energy of the photon matches an excitation in the system. So the hydrogen atom can absorb a photon if its energy matches one of the frequencies in the hydrogen spectral series.

A proton is a composite object and it does have a spectral series. However the excited states of the proton involve rearrangements of the energy and angular momentum of the partons that make it up. The transition energies are in the 100MeV to 1GeV range so they are far beyond anything that could be excited by visible light. However gamma ray photons could excite such transitions and be absorbed.

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  • $\begingroup$ However gamma ray photons could excite such transitions and be absorbed. Would then the photons be emitted back soon, just like in the case of electrons? $\endgroup$
    – Xfce4
    Oct 3, 2021 at 5:23
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    $\begingroup$ @Xfce4 Yes, it would. Radionucleides that emit gamma rays are doing exactly this. $\endgroup$ Oct 3, 2021 at 5:26
  • $\begingroup$ Thank you. But "it would" sounds like it is optional, is it? If not how long does it take for the photon to be emitted back? $\endgroup$
    – Xfce4
    Oct 3, 2021 at 6:40
  • $\begingroup$ @Xfce4 The lifetime of the excited state is extremely variable and depends on the nucleus and the state. It can be anything from days to femtoseconds. When the photon is absorbed the energy has to go somewhere. It could cause the nucleus to fission or eject one or more nucleons, in which case the energy would be released as the kinetic energy of the decay products. If this doesn't happen the only other option is for the energy to be released as a new photon. $\endgroup$ Oct 3, 2021 at 6:48
  • $\begingroup$ Thank you very much. If this doesn't happen the only other option is for the energy to be released as a new photon. Do you mean that the response of the nucleus is random or there is an order, i.e. if the energy of the photon is enough for the fission the fission occurs 100%? Is it possible that a sufficiently high energy photon can be absorbed and emitted back without resulting in a fission? $\endgroup$
    – Xfce4
    Oct 3, 2021 at 7:08
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Does a proton have a "bandgap"? If yes, what happens when a photon is absorbed by a proton?

For single protons, as in a plasma , there exists Compton scattering .

The photon transfers part of its energy to the proton and scatters off at a lower energy/frequency, the proton taking up the energy-momentum balance. This is a continuous spectrum, from very low energies on.

For high energy photons the quark structure of the proton can be probed and depending on the energy more particles are created by the interaction with the constituent quarks. There are no energy levels for the quarks within the proton.

There do exist baryonic resonances, i.e. with the same three quarks at higher excited states. If one were to scatter gammas of the appropriate energy, some resonances will be excited , for example the N(1520) decays into a proton and a gamma so in the crossection a resonance will be seen passing that energy of center of mass in proton gamma scattering.

This is a theoretical study of gamma proton scattering at high energies.

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The type of interaction depends on the energy of the photon, based on the Klein-Nishina formula.

From Wikipedia:

  • Very low energy photons (visible light; as long as the photon energy is much less than the mass energy of the particle, i.e. Compton wavelength) yields Thompson scattering, which is elastic scattering with electrons.
  • Low energy photon (a few eV to a few keV, corresponding to visible light through soft X-rays) can eject an electron from its host atom entirely (a process known as the photoelectric effect), instead of undergoing Compton scattering.
  • High energy photons (comparable to the rest energy of the electron, 511 keV) bombardment results in the Compton scattering: the electron being given part of the energy (making it recoil), and a photon of the remaining energy being emitted in a different direction from the original, so that the overall momentum of the system is conserved. If the scattered photon still has enough energy, the process may be repeated. In this scenario, the electron is treated as free or loosely bound.
  • Higher energy photons (1.022 MeV and above) may be able to bombard the nucleus and cause an electron and a positron to be formed, a process called pair production.
  • Very high energy photons (hundreds of MeV) may yield Nuclear Compton scattering.
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The proton has to be absorbing and emitting photons for it be exerting an electrical force on the electron. Most of those are virtual photons, but clearly the proton is capable of absorbing a photon for some value of "photon" and "absorb".

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