Lower kinetic temperature implies population inversion?

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Discussion Overview

The discussion revolves around the relationship between lower kinetic temperatures and the occurrence of population inversion, particularly in the context of astrophysics and molecular clouds. Participants explore the conditions under which population inversion can occur and the processes involved, while questioning the generality of the claim regarding low kinetic temperatures.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants question the assertion that lower kinetic temperatures imply population inversion, suggesting that additional context is necessary regarding the specific materials and conditions involved.
  • One participant provides an example of a low density sample of OH molecules in interstellar space, indicating that not all low kinetic temperature scenarios lead to population inversion.
  • Another participant emphasizes the need to clarify what is meant by population inversion and the processes that could lead to it, such as infrared pumping from nearby astronomical sources.
  • A later reply references specific literature discussing the conditions for population inversion related to the 21 cm line and describes the pumping processes involved, particularly in the context of hydrogen atoms and their hyperfine states.
  • One participant notes that at high temperatures, the 21 cm line is observed as thermal emission, complicating the demonstration of population inversion, while at low temperatures, thermal excitation is less likely, making pumping processes more relevant.

Areas of Agreement / Disagreement

Participants generally do not agree on the implications of lower kinetic temperatures for population inversion, with multiple competing views and a lack of consensus on the specific conditions and processes involved.

Contextual Notes

Participants highlight the importance of defining the specific conditions under which population inversion is considered, as well as the processes that may lead to it, indicating that the discussion is limited by these unresolved aspects.

TheCanadian
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I was just wondering: why does lower kinetic temperature imply population inversion? I've read this is true in a few (astrophysics) texts regarding molecular clouds, although this seems to be a more general result that I haven't really read an entire explanation for. Nevertheless, do you have any idea on why low kinetic temperatures imply population inversion?
 
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I think there is some context missing. Not every material with a low kinetic temperature has a population inversion (of what?).
 
mfb said:
I think there is some context missing. Not every material with a low kinetic temperature has a population inversion (of what?).

An example would be a low density sample (##\sim10^8## molecules/cm##^3##) of molecules such as OH in interstellar space.
 
There is still something missing.
Population inversion of what, based on which process?
 
mfb said:
There is still something missing.
Population inversion of what, based on which process?

To clarify an article where I read this from, it's in this paper which states:

Whether or not a population inversion can easily be realized for the energy levels leading to the 21 cm line, it has been considered in the existing literature (Shklovskii 1967; Storer & Sciama 1968; Dykstra & Loeb 2007), and we know of at least one region (the Orion Veil) where the kinetic temperature is lower than the 21 cm spin temperature, providing evidence for a population inversion (Abel et al. 2006). The main pumping process covered in the literature corresponds to the situation when a H I gas is close to a source of radiation that emits a field with an intensity Iv (n) in the neighborhood of the Lyα line. A hydrogen atom in the ground hyperfine state (n = 1, F = 0) can absorb a photon and become excited to the n=2 level. Later on, the atom returns to the upper hyperfine state (n = 1, F = 1), emitting a slightly less energetic photon than the initial one absorbed by the atom. The same can happen for a hydrogen atom initially in the hyperfine state (n = 1, F = 1) that returns to the ground (n = 1, F = 0) state after excitation to the n=2 level, emitting a slightly more energetic photon in the process. The absorption rate of the photons for both cases depends on the intensity of the radiation Iv (n), but the return (emission) process does not.
 
As far as I understand, it is just about experimental observation. If the temperature is high, you will see this 21 cm as thermal emission, and it gets hard to show population inversion. If the temperature is very low, on the other hand, thermal excitation is ruled out, and pumping processes are the most likely source.
 

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