Collision-induced free-free absorption in fluids and superfluids

In summary, when two different lone atoms collide, free-free emission is allowed. When two identical lone atoms collide, triple collisions do not have the symmetry of two atom collisions, meaning that three atoms colliding can undergo free-free absorption into translational modes, and pure monoatomic gases should have infrared absorption proportional to cube of density.
  • #1
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TL;DR Summary
How much do dense fluids absorb IR when tenuous gases cannot?
Lone homonuclear diatomic molecules have vibrational excitations and rotational excitations. However, due to lack of transitional dipole moment, these are strongly forbidden to absorb IR. Lone atoms don´t have the above excitations in the first place.
Now, when a diatomic molecule collides with another molecule or atom, it means that 1) the symmetry is lifted so absorption to vibrational and rotational excitations should be allowed, and 2) there is the possibility to excite pure translational, free-free movement. Causing collisional induced absorptions proportional to square of density.
What happens when a lone atom collides with another lone atom? No vibrational excitations, nor rotational. Free-free translational movement exists, but when the two lone atoms are identical, there is no transitional dipole moment, so forbidden.
Turns out that when two different lone atoms collide, free-free emission is allowed.
As for identical lone atoms: turns out that triple collisions do not have the symmetry of two atom collisions, meaning that three atoms colliding can undergo free-free absorption into translational modes, and pure monoatomic gases should have infrared absorption proportional to cube of density.

So far I was discussing fluids where the states are free and not particularly affected by Bose condensation or Pauli ban.
But what happens to free-free, triple-collision IR absorptivity when the fluid becomes a superfluid?
Does He have appreciable double collision absorptivity? Not natural He of course (He-3 fraction is tiny). He is miscible in all proportions over about 1 K and appreciably soluble to zero, but He-3 and 4 might not be all that different for the transition dipole moment...
 
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  • #2
snorkack said:
Now, when a diatomic molecule collides with another molecule or atom, it means that 1) the symmetry is lifted so absorption to vibrational and rotational excitations should be allowed, and 2) there is the possibility to excite pure translational, free-free movement. Causing collisional induced absorptions proportional to square of density.
What happens when a lone atom collides with another lone atom? No vibrational excitations, nor rotational. Free-free translational movement exists, but when the two lone atoms are identical, there is no transitional dipole moment, so forbidden.
Can you please provide citations to the literature that support these conjectures?
 
  • #4
Thank you, but I asked for citations to the literature, not the auto-launched download of an 11.5MB pfd file.
 
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  • #5
The linked document is a thesis:
Classical molecular dynamics
simulations of collision-induced
absorption: method development
and evaluation
Wissam Fakhardji
Department of Engineering Sciences and Mathematics
Division of Materials Science
Lule ̊a University of Technology
Lule ̊a, Sweden
Funnily, the nominal year of defence/publication is not prominent.
Publications included in this thesis:
(A) – W. Fakhardji, P. Szab ́o, M.S.A El-Kader, A .Haskopoulos, G. Maroulis, and
M. Gustafsson, “Collision-induced absorption in Ar–Kr gas mixtures: A molecular
dynamics study with new potential and dipole data”, The Journal of Chemical
Physics, vol. 151, 144303 (2019); doi:10.1063/1.5099700
(B) – W. Fakhardji, P. Szab ́o, M.S.A El-Kader, and M. Gustafsson, “Molecular
dynamics calculations of collision-induced absorption in a gas mixture of neon and
krypton”, The Journal of Chemical Physics, vol. 152, 234302 (2020); doi:10.1063/5.000618
(C) – W. Fakhardji, P. Szab ́o, and M. Gustafsson, “Direct method for the MD
simulations of collision-induced absorption: application to an Ar–Xe gas mixture”,
In preparation
 
  • #6
snorkack said:
... 2) there is the possibility to excite pure translational, free-free movement. Causing collisional induced absorptions proportional to square of density.

But what happens to free-free, triple-collision IR absorptivity when the fluid becomes a superfluid?

I have not had the opportunity to scrutinize the thesis, but please allow me to ask for clarification of a couple of points.

1. Are you asking about discrete states or continuum phenomena?

2. Can you clarify the concept of "free-free transitions"? The terminology "free-free transitions" is well established and describes a specific physical interaction. That being the acceleration of free electrons via Coulombic interactions with positive ions. The trajectory of the electron is altered by the acceleration with a change in the kinetic energy of the free electron with concomitant emission or absorption of a photon. This is a well known and well characterized physical phenomenon typically associated with spectroscopic continua. It is not obvious to me how that physics is applicable to a superfluid environment.

3. Can you clarify the concept of "excite pure translational movement"? Are you suggesting that discrete, quantized translational states exist? If not, how does this differ from normal momentum transfer?
 
  • #7
Hyperfine said:
1. Are you asking about discrete states or continuum phenomena?
Continuum phenomena. I only mentioned discrete states for context, hoped you get it.
Hyperfine said:
2. Can you clarify the concept of "free-free transitions"? The terminology "free-free transitions" is well established and describes a specific physical interaction. That being the acceleration of free electrons via Coulombic interactions with positive ions. The trajectory of the electron is altered by the acceleration with a change in the kinetic energy of the free electron with concomitant emission or absorption of a photon. This is a well known and well characterized physical phenomenon typically associated with spectroscopic continua.
Is "free-free transition" limited to electron-positive ion interaction?
For simple examples:
If you have OH radical, it consists of 2 significantly different atoms. So symmetry does not forbid dipole moment, and the OH radical stretching can freely emit infrared. Which is lines.
So are rotational lines of OH radical.
Now, you can have bound-free or free-bound transitions of O-H. Such as, neutral O atom and neutral H atom collide, form OH radical and emit their binding energy+their translational energy as one photon. This is allowed (same reason why stretching vibrational radiation is allowed) and continuum (because of the translational energy added).
And you can also have a collision where a free neutral O atom and a free neutral H atom collide with each other, separate but inelastically - either emitting a photon and separating with lower translational energy, or absorbing a photon and separating with increased translational energy. This is allowed because although both are neutral atoms, they are different ones. What do you call it if not "free-free transition"?
Hyperfine said:
It is not obvious to me how that physics is applicable to a superfluid environment.
It is applicable to fluids because 1) liquids are dense, so they are likely to have a lot of collisions and thus a lot of collision induced absorptions and emissions, but 2) in liquids, including superfluids, in contrast to solids, translational states are not quantized.
Hyperfine said:
3. Can you clarify the concept of "excite pure translational movement"? Are you suggesting that discrete, quantized translational states exist? If not, how does this differ from normal momentum transfer?
If two neutral atoms or molecules undergo elastic collision, they transfer momentum, but in their centre of mass frame, their momenta merely change sign and energies are unchanged. In a different frame, their combined energy still is unchanged.
What I mean is inelastic collision. Two atoms or molecules collide, their internal states are unchanged before and after collision, but their translational energy has changed because of either emission or absorption of a photon at collision.
 
  • #8
snorkack said:
Is "free-free transition" limited to electron-positive ion interaction?
Yes. The terminology describes Coulombic interactions between free charged particles. It is Bremsstrahlung. Misappropriation of accepted nomenclature lacks good quantum numbers. :wink:
snorkack said:
It is applicable to fluids because 1) liquids are dense, so they are likely to have a lot of collisions and thus a lot of collision induced absorptions and emissions, but 2) in liquids, including superfluids, in contrast to solids, translational states are not quantized.
How do those fluid environments support a plausible density of free charged particles?
snorkack said:
What I mean is inelastic collision. Two atoms or molecules collide, their internal states are unchanged before and after collision, but their translational energy has changed because of either emission or absorption of a photon at collision.
In quantum systems, the accepted definition of an inelastic collision is one that changes the quantum states.

Again, the collisionally induced change in translational energy is not best characterized as a transition.
 
  • #9
Hyperfine said:
Yes. The terminology describes Coulombic interactions between free charged particles. It is Bremsstrahlung. Misappropriation of accepted nomenclature lacks good quantum numbers. :wink:
Then how do you describe interactions between free neutral particles which stay neutral but emit or absorb photons?
Hyperfine said:
How do those fluid environments support a plausible density of free charged particles?
As I mentioned - interactions between free neutral particles.
Hyperfine said:
In quantum systems, the accepted definition of an inelastic collision is one that changes the quantum states.

Again, the collisionally induced change in translational energy is not best characterized as a transition.
But the process includes creation or destruction of a photon.
 
  • #10
snorkack said:
Then how do you describe interactions between free neutral particles which stay neutral but emit or absorb photons?
By articulating explicitly what you are discussing. Not by taking over established terminology that describes a distinct and different phenomenon.
snorkack said:
As I mentioned - interactions between free neutral particles.
But those are not "free-free" interactions according to accepted nomenclature.
snorkack said:
But the process includes creation or destruction of a photon.
To the best of my knowledge, the term inelastic collision when applied to a quantum system means that there is a change in quantum state of either or both collision partners. The photon is not relevant. Nor is it obvious how you get a photon without a change in quantum state unless you are merely talking about a photon required to conserve the total energy of the collision. If so, what is new about that?

All that I am suggesting is that you adopt the established terminology accepted by the physics community. If nothing more, it certainly tends to preclude substantive confusion on the part of your readers.

In your post (#7) above you apparently want to consider collisions between O and H atoms within a dense fluid environment. Highly unlikely! Both of those species are sufficiently chemically reactive to react with the first thing they encounter.
 
  • #11
Hyperfine said:
By articulating explicitly what you are discussing. Not by taking over established terminology that describes a distinct and different phenomenon.

But those are not "free-free" interactions according to accepted nomenclature.
Check multiple places in my linked work. Like page 113 by PDF (in one of the articles). "Free-free" is used naturally to describe collision between neutral atoms resulting in emission/absorption. I was applying established nomenclature, not "taking it over".
 
  • #12
I could counter with the quotation below, but why bother? Obviously this discussion is fruitless.
Oxford Dictionary of Astronomy: Free-free transition: The emission or absorption of radiation by an electron that is unbound to an atom before and after the event. As an electron passes an ionized atom it can be accelerated or decelerated, respectively absorbing or emitting a photon in the process. Since the photon can be of any wavelength, radiation emitted in this way has a continuous spectrum. The radiation emitted in this way is a type of thermal radiation, typically emitted by a hot plasma, and is also known as thermal bremsstrahlung. It is seen in many emission nebulae.
 
  • #13
Thread closed temporarily for Moderation...

Update -- thread is reopened for now.
 
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  • #14
Some classic experimental work:
https://upload.wikimedia.org/wikipe...uefied_gases_(IA_farinfraredabsor390jone).pdf
If citation is what you want... Seems to be NBS Technical Note 390 from 1970. Far Infrared Absorption in Liquefied Gases. Some M. C. Jones.
Page 21...23 discusses a far IR band of ortho-diprotium, (around 100/cm wavenumber, 0,15/cm intensity), and refers to assigning it as translational in same author´s other work. Page 33 discusses liquid Ar: no absorption found as expected, but reachable intensities only extend to maybe 0,05/cm intensity.

Pure monoatomic fluid cannot have dipole in double collisions, but can in triple collisions. The translational bands for triple collision should show up at lower intensities than were observable in the above work... somewhere. Where?
 

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