Is liquid helium composed of both ortho and para-helium?

Creator

In a batch of liquid helium (say < 8* K.) is there a mixture of both ortho and para-helium, or is it only ground state para-helium.?
I was under the impression that ortho was a meta-stable state which cannot decay to ground state Para-helium by radiative emission, but by meta-stable we are talking only a fraction of a second...right?

What am I missing here.?

Anyone?
..

alxm

IIRC, the singlet-triplet splitting is quite large, ~20 eV. So it should be entirely in the ground state, unless there are special conditions going on (they've made triplet-helium BECs).

It can decay radiatively, just not by a single-photon process.

Creator

Thanks alxm...
What special conditions are you referring to ? How do I create triplet state experimentally?

So if triplets decay after 1/10 second (is that about correct?) then by what process do they decay?

alxm

Well I was thinking if you specifically generate triplet helium and then cool it in an optical trap or similar. Not exactly your usual state of affairs.

This is a 'forbidden' transition; It can only occur (in an isolated atom) due to two-photon processes, which one can think of as a decay to a 'virtual' level in-between the two states. For Helium it's not symmetric; one of the photons carries more energy than the other, so you end up with two peaks, one at ~70 nm and one at ~2400. (See e.g. this paper, if you want some details)

1/10 of a second.. I'm not sure. An isolated ³He atom would probably have a lifetime orders of magnitude longer. But with gaseous or liquid helium, in a container, etc, you have many more interactions that can go on and assist the process. (For instance formation of a He₂ molecule in the [tex]^3\Sigma_u^+[/tex] state.)

Creator

That was my other main concern: By what method would you "specifically generate triplet helium"??(not BEC)


Thanks for the G W Drake link. I'm not too sure I understand it all....Of course, Selection rules prohibit single photon transitiion .... so it goes two photon route...OK, but this rate is extremely low, right?
I have read elsewhere that the most probable decay is thru 'collisional' process, I am not sure what that refers to ...any info there?

GW Drake has a later article in which it appears as if he is saying that there is a MORE probable Magnetic dipole transition to ground state decay...am I reading that right?
See here:

http://cos.cumt.edu.cn/jpkc/dxwl/zl/...atomic/099.pdf


Creator

alxm

I haven't researched it, but off the top-of-my head you could simply irradiate the helium with a wavelength that excited it from the singlet ground state to a state that was able to decay to the triplet state. Given that that state has a relatively long lifetime, this should give you all the triplet helium you want, given that the radiation is sufficiently intense.

Yes, that's the most probably situation for a non-isolated helium, and in practice the most common decay method. The easiest and most common is simply that two triplet-helium atoms collide and exchange an electron (or viewed another way: flip each other's spins). That process is fairly straightforward.

It appears so. Guess I was wrong. Given that it appears Breit and Teller also thought the two-photon process was more likely, at least I'm in excellent company!
Both forms of decay are certainly many orders-of-magnitude less significant than the collision route, in a 'normal' environment. This is all mostly of astrophysical interest, since space is full of helium atoms in high vacuum.

Creator

Thanks again, Alxm. and sorry for the late response.

The collisional process is interesting...apparently, collision with a container wall can also do the trick.
1st question: At what temperature is there enough kinetic energy to spin flip each He triptlet state? Probably somewhere there is a temperature dependent rate formula?
2. Can collisional spin flip be by mecahnical means, (ex,rotation of the fluid) ?



After further reasearch it appears as though I was wrong about the 1/10 sec. excited lifetime.
The He(2S^3) state lifetime is about 8000 seconds!! That's a quantum eternity, and surprising...probably the longest"Meta-stable" state around.....but makes sense knowing the fact that it is radiatively forbidden transition to ground.

Creator