What Happens to Photons During Ionization?

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Discussion Overview

The discussion centers on the fate of photons during the ionization of atoms, exploring the mechanisms involved in photon absorption, energy transfer, and the implications of photon energy relative to atomic states. The scope includes theoretical considerations and conceptual clarifications related to ionization processes and photon interactions with matter.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that when a photon is absorbed for ionization, it is annihilated, with its energy contributing to the kinetic energy of the ejected electron.
  • Others argue that if the photon energy exceeds the ionization energy, the excess energy becomes kinetic energy of the electron.
  • A participant notes that in the case of the photoelectric effect, the photon energy is fully absorbed, and no secondary electron is produced.
  • There is a discussion about photons that have energy falling between two quantum states, suggesting that such photons cannot be absorbed due to a zero absorption cross section.
  • Another participant introduces the concept of Compton scattering, where photons can inelastically scatter off bound electrons, potentially leaving them in higher bound states rather than being fully ejected.

Areas of Agreement / Disagreement

Participants express differing views on the outcomes of photon interactions during ionization, particularly regarding the absorption of photons with energies that do not match specific quantum states. The discussion remains unresolved with multiple competing perspectives on the mechanisms involved.

Contextual Notes

Limitations include assumptions about the energy levels of photons and electrons, as well as the specific conditions under which different interactions (e.g., photoelectric effect vs. Compton scattering) occur. The discussion does not resolve the complexities of these interactions.

Thermodave
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If a photon is used to ionize an atom then what happens to the photon? If the wavelength of the photon corresponds to the ionization energy, then the photon is absorbed/annihilated, yes? What if the photon has more energy. Is a lower energy photon emitted during ionization?
 
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Yep, the photon would be absorbed (annihilated). The energy that was in the photon would go into releasing the electron, and if there were any energy left over, it becomes kinetic energy of the electron. So the more energetic the photon, the more energetic (faster-moving) the electron that comes out.
 
If the photoemission is via the photoelectric effect, there is no secondary electron. The photon energy is 100% absorbed. If there is ionization, there are usually atomic emission x-rays etc. If the photon interaction is via Thomson scattering, there is a secondary electron due to photon scattering..
Bob S
 
Thank you for the answers. However, there was one other point I was getting to. If the photon has an energy such that, if absorbed by the atom, it falls "between" two quantum states, then this photon cannot be absorbed, correct? Since the extra energy can't disappear, the absorption cross section is zero?
 
Photons can scatter inelasitically off of bound electrons in the atom. This is Compton scattering, usually leading to an ejected electron, but occasionally the electron could end up in a higher bound quantum state. One example might be a photon with an energy exceeding 0.75 x 13.6 eV inelastically scattering off of the bound electron in a hydrogen atom, leaving the electron in a higher p state. Because two photons are involved (one incident, one leaving), the cross section (probability) is lower.
Bob S
 

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