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snorkack
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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...
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...