Anti-hydrogen molecular spectra?

[Mark Harder]

I'm not a molecular physicist, so my speculations are bound to be somewhat naive. I'm only hoping to initiate some discussion around the subject, which I thought might be interesting to all.
Now that the electronic absorption spectrum of the atomic anti-hydrogen 1s->2s electronic transition has been measured (Nature (2016) doi:10.1038/nature21040 Received 29 November 2016 Accepted 07 December 2016 Published online 19 December 2016 ), I wonder what folks think about the possibility of doing a similar series of experiments on anti-H₂.
In the above experiment, atomic anti-H was held in a magnetic trap for ~17 sec while the electronic absorption spectrum was collected. The spectrum agreed with that of ordinary H to one part in 10¹⁰. If diatomic anti-H could be created, could it also be trapped magnetically,(necessary to hold the antimatter away from ordinary matter of the apparatus), or does that phase of the experiment require the unpaired positron in the anti-atom? Assuming that trapping could be accomplished, and the molecules could be produced, in sufficient quantity to be observed, one would have a 4-body anti-matter system to probe. I would think that the molecule would display positron-positron, antiproton-antiproton, and two types of positron-antiproton interactions, all of which would contribute to the range of molecular orbital transitions that could be observed by means of UV-visible spectroscopy. Also, wouldn't infrared spectroscopy probe the vibrational stretching transitions, the details of which reflect the coulombic repulsion of the nuclei and the stabilization of the molecule through the delocalization of the 2 positrons? A more complex system, to be sure, but therefore one likely to yield more answers.

 

[DrClaude]

The difficult part is actually creating the anti-H₂ molecule, and at low enough temperatures such that they can be trapped. For the formation of dimers, enough anti-H has to be produced to get reasonable collision rates, and in addition you need a mechanism to for the molecules. What complicates things is also the fact that it is difficult to build lasers operating at the right frequencies for hydrogen. My hunch is that people will try to work with anti-H₂⁺ first, which you can trap more easily.

Once you have molecules, researchers will of course try to probe them in all ways possible, in the search for any asymmetry between regular and anti-matter.

 

[Mark Harder]

Thanks for that, Dr Claude. BTW, I wonder if by "...work with anti-H₂⁺ first...", you meant to refer to anti-H₂^- (the missing positron removing one + charge)? As I recall from P.Chem, it's easier to solve the S.E. for H₂⁺ than the ordinary neutral H₂ molecule. Can these (pro- or anti-) be produced handily? Just curious. It never occurred to me that such a beast could actually be made and probed in the flesh, so to speak.

 

[BvU]

Use brackets ! anti-(H₂⁺) = (anti-H₂)^-

 

[Vanadium 50]

What do you learn with molecules that you don't learn from atoms?

 

[mfb]

What do you learn from antihydrogen that you don't learn from positronium?

Antihydrogen instead of positronium leaves some room for positron/antiproton interactions, antihydrogen molecules would add long-range antiproton-antiproton interactions, and positron-positron interactions for neutral molecules. That would be extremely odd, sure, but new effects might hide at unexpected places.

 

[Vanadium 50]

What do you learn from antihydrogen that you don't learn from positronium?

That's a very good question. I would say that because it lives longer, you are in principle sensitive to smaller energy shifts than with positronium. This probably isn't true, because it is so much easier to create positronium we probably have a better handle on it because of sheer numbers.

 

[mfb]

The finite lifetime is certainly a point once antihydrogen spectroscopy gets more sensitive. This paper claims 10^-12 precision for positronium is possible, the antihydrogen results don't have this accuracy yet.