Complete Measurement of Molecular Breakup

Suidae writes "PhysicsWeb is reporting that physicists have made a 'complete' measurement of the break-up of a molecule for the first time. Reinhard Dörner of the University of Frankfurt and co-workers in Germany, the US, Australia and Spain recorded the two electrons and two nuclei that were released when a single photon split a molecule of deuterium into its basic components. The experiment could lead to a better understanding of many physical and chemical processes through improved knowledge of the quantum dynamics of many-particle systems."

Comments

Confusing

[Micro$will]

Deuterium is an isotope of hydrogen, not a molecule.

FP BTW.

Re:Confusing

[dougmc]

I agree, it's confusing.

However, hydrogen (and deuterium, which is just hydrogen with an extra neutron) atoms are almost never found alone. They're found in molecules of two hydrogen atoms. I think that's what they're talking about breaking up.

Re:Confusing

[dougmc]

And to expand on my other post in this thread, they're not breaking the nucleus up at all -- just knocking the two deuterium atoms apart.

75 eV isn't nearly enough to split the nucleus apart anyways.

I wonder why they used deuterium molecules rather than just hydrogen, though. I understand why they used hydrogen -- a hydrogen-hydrogen molecule is the simplest possible molecule -- but why deuterium?

Let me see if I can answer my own question ...

Deuterium has somewhat different chemical properties than hydrogen due to the different nuclear mass (classically, it's a two body problem, look up `reduced mass' if you want more details.) With a larger nucleus mass, the electrons would have a lower average kinetic energy, and by the virial theorem the average potential energy would therefore be higher (KE = - 1/2 PE) (a smaller absolute value, but it's negative, so it's higher) and therefore less tightly bound.

So, I'm guessing they used deuterium molecules instead of hydrogen molecules because it would require less energy to split them apart. They're making things as easy as possible ...

Re:Confusing

[Meridun]

D2 is the diatomic molecule for deuterium gas, which is the normal state of deuterium at STP (standard temperature and pressure).

It's sometimes hard for people to bear in mind, but isotopes actually DO behave almost identically to each other in most non-nuclear chemical reactions.

I found it very interesting when I was doing NMR (nuclear magnetic resonance; used to help determine molecular structures) work in the labs, we had to use deuterated solvents, so that they would resonate differently and therefore could be removed from the resulting data. Therefore, I would use D2O (deuterated water) and other common solvents that had all the hydrogens replaced with deuterium. Aside from the very expensive pricetag, you'd never have known the difference.

Re:Confusing

[Cecil]

Despite being the worst writeup EVER (Hello, could we get this submitted by someone who has more than a 1st grade science class under their belt?) after reading the article it's clear what they were doing.

You seem to be confusing the meaning of 'nuclei' with 'subatomic particle'. Nuclei is the plural form of nucleus. They're talking about two seperate "nucleuses" which are flying apart, because both have an overall positive charge thanks to the proton, so both repel each other.

To clarify what actually happened here: They started with a single molecule of deuterium. A molecule of deuterium is simply two deuterium atoms (deuterium is just like hydrogen, only it has a neutron in the nucleus as well) in a covalent bond, the exact same way hydrogen is normally found as an H2 molecule.

Then they shot a photon at their molecule, which knocked the two electrons being shared by the two atoms away. Since the sharing of electrons is what causes a covalent bond to be happen in the first place, the bond breaks up, and they measure exactly what happened.

Deuterium poisoning.

[Christopher Thomas]

But curiously, drinking deuterated water is apparently poisonous.

This is because the chemical behaviors of deuterium and light hydrogen are slightly different.

You can think of the electron and nucleus co-orbiting about a common centre of mass, rather than the electron orbiting while the nucleus remains fixed. Where the point is depends on the ratio of the masses of the electron and the nucleus (about 2000:1 for light hydrogen, and about 4000:1 for deuterium). The different orbit radius (for any given energy) for each case means that the energy level at which the orbit circumference is an integer number of electron wavelengths will be different for deuterium and light hydrogen.

This means that the energy structure of the electron shells is slightly different, which means that they will behave slightly differently chemically. This fact is exploited in some of the methods of isolating heavy hydrogen from light hydrogen (electrolysis method, as the reduction potential is different, and the more common chemical method involving forming hydrogen sulphide, as the rates of reaction are different).

In the case of ingestion, deuterium's chemical behavior is similar enough to that of hydrogen that it gets incorporated into chemicals and otherwise interacted with as hydrogen would be, but different enough that it mucks up some of these reactions. Result, poisoning, much as you get from heavy metals displacing their chemical analogues (though less so, because D and H are a lot more similar, and your body cycles hydrogen through itself pretty quickly, while metals tend to accumulate).

As far as hydrogen isotopes go, though, tritium is the main concern. It's a beta emitter, and is formed in water-cooled reactors (especially the heavy-water-moderated reactors Canada uses, as only one transmutation step is required instead of two). It's a very low-energy emitter, but if ingested, will still cause problems. It's less nasty than most contaminents, though, as hydrogen gets cycled through the body very quickly, and tritium has a half-life of about a decade (short enough to disappear within a lifetime, long enough that it cycles out of the body without depositing much of its radiation dose).

Deuterium is mildly chemically toxic, but is not radioactive.

How does this stuff get modded up?

[mcmonkey]

Deuterium is an isotope of hydrogen, not a molecule.

That's like saying the iMac is an Apple, not a computer. Yes, Deuterium is an isotope of hydrogen. That does not preclude Deuterium molecules. Actually, it predicts it. We get D2 just like we get H2. But if you want to get that FP (and first mod) you can't let the facts get in the way.

A deuterium nucleus has a proton and a neutron... only one of these has a positive charge.

How is that 'interesting'???? Again, while technically true, that statement shows a complete ignorance of the discussion at hand. A proton has a positive charge. A nucleus of one proton and one neutron also has an overall positive charge. When you have two such nuclei they are both positively charged.

Okay? Get it? There's nothing confusing about it; nothing weird. Two deuterium atoms come together to form a molecule. Each nucleus has a positive charge. They're breaking up a molecule, not an atom or nucleus.