# A Lower kinetic temperature implies population inversion?

1. Dec 31, 2016

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?

2. Jan 1, 2017

### Staff: Mentor

I think there is some context missing. Not every material with a low kinetic temperature has a population inversion (of what?).

3. Jan 1, 2017

An example would be a low density sample ($\sim10^8$ molecules/cm$^3$) of molecules such as OH in interstellar space.

4. Jan 1, 2017

### Staff: Mentor

There is still something missing.
Population inversion of what, based on which process?

5. Jan 1, 2017

It could be infrared pumping from a nearby protostar or supernova remnant.

6. Jan 3, 2017

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.

7. Jan 3, 2017

### Staff: Mentor

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.