Negative Hydrogen Ions in Cyclotrons and elsewhere

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Negative Hydrogen Ions in Cyclotrons and elsewhere

I was vaguely aware that negative hydrogen ions are used in some cyclotrons and this improves beam current.

I've been thinking about this recently after noting that hydrogen is actually quite electronegative in comparison to most metals (just less electronegative than oxygen and other things it tends to bind with).

So, presumably, if you mix hydrogen gas with electrons in a vacuum then they will tend to make H2- molecular ions. Is that right?

It sets me wondering on a few things.

Presumably, the energy of a H2- ion is +13eV?

Does it radiate the Balmer series of photons as a second electron falls into that first shell, and does anything else happen if it let's an electron go?

What is the activation energy needed to prompt it to shed the electron?

If hydrogen molecules and electrons are floating around in a vacuum with, say, 1eV electrons, do they tend to form H-, H2-, H3- and if all of the above, then what is the relative proportion of species?

Presumably if H2- meets more electrons it might break up and form two H- ions?

I can't find any texts at all about this, so if there is something then please let me know.

 

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You are mixing hydrogen molecules and isolated ions in your post, that is confusing.

I was vaguely aware that negative hydrogen ions are used in some cyclotrons and this improves beam current.

Do you have some examples? Tandem accelerators use H^- where the electrons are removed at the positive terminal to use their acceleration voltage twice, but that is not what you are talking about.

Presumably, the energy of a H2- ion is +13eV?

Do you mean $H_2^-$, the molecule?
Do you mean $H^{2-}$, a hydrogen atom with three electrons? How would a negatively charged hydrogen bind yet another electron?
Why would you expect 13 eV to appear anywhere - and relative to what?

Does it radiate the Balmer series of photons as a second electron falls into that first shell, and does anything else happen if it let's an electron go?

Second electron? Are you now talking about $H^-$? Its energy levels are different as you can't neglect the changed potential from the first electron.

If hydrogen molecules and electrons are floating around in a vacuum with, say, 1eV electrons, do they tend to form H-, H2-, H3- and if all of the above, then what is the relative proportion of species?

Please clarify your notation.

 

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You are mixing hydrogen molecules and isolated ions in your post, that is confusing.

Sorry, what I meant for all of the above was one -ve charge on each of those, so (H2)- and (H3)-. I presume you can't have a (H2)2- because it'd just fall apart as 2 x H-.

Do you have some examples? Tandem accelerators use H^- where the electrons are removed at the positive terminal to use their acceleration voltage twice, but that is not what you are talking about.

See at 0'14"

Do you mean $H_2^-$, the molecule?

Yes

Do you mean $H^{2-}$, a hydrogen atom with three electrons? How would a negatively charged hydrogen bind yet another electron?

No, I wouldn't expect that so presumed you'd see that is what I meant.

Why would you expect 13 eV to appear anywhere - and relative to what?Second electron?

Because if it takes 13eV of energy to shove an electron off an H atom then presumably it'd take that to put it back, and another 13eV to put another one there? I don't know, that's why I am asking.

Are you now talking about $H^-$? Its energy levels are different as you can't neglect the changed potential from the first electron.

I am not sure I understand your response 'you can't neglect the changed potential'? I am assuming that if an electron drops from 'elsewhere' into the 1st shell then energetically speaking it'd be the same for the second electron as the first, but that might be completely wrong so that is why I am asking.

Please clarify your notation.

(H)- (H2)- (H3)-

 

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See at 0'14"

They use H- because it makes the extraction of the beam from the cyclotron easy: Remove the electrons and the accelerated proton leaves the accelerator cleanly. With H+ you would need a different magnetic field at some point, then your magnetic field gets inhomogeneous and you run into all sorts of trouble.

Because if it takes 13eV of energy to shove an electron off an H atom then presumably it'd take that to put it back, and another 13eV to put another one there? I don't know, that's why I am asking.

13 eV is the binding energy of a single electron to a single proton. A second electron has a different binding energy already, even with a single proton, because you cannot ignore the repulsion between the electrons. A hydrogen molecule has completely different energy levels.

(H)- (H2)- (H3)-

Their ratio would depend on the conditions of the hydrogen.

 

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So, are there answers to my above? Does this information exist anywhere or is it just handwavy physicists talk?

 

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So, are there answers to my above?

My previous post?

Does this information exist anywhere or is it just handwavy physicists talk?

Make handwavy questions, get handwavy answers.

 

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My previous post?Make handwavy questions, get handwavy answers.

Thanks but somewhat unhelpful to my very-not-handwavy questions, which I'll pinpoint again;-

Does it radiate the Balmer series of photons as a second electron falls into that first shell, and does anything else happen if it let's an electron go?

(And with your further point that it is not the same as the first electron, then is it another photon series, or just one wavelength, and if so what is that?)

What is the activation energy needed to prompt it to shed the electron?

If hydrogen molecules and electrons are floating around in a vacuum with, say, 1eV electrons, do they tend to form H-, (H2)-, (H3)- and if all of the above, then what is the relative proportion of species?

Presumably if (H2)- meets more electrons it might break up and form two H- ions?

And as you've now helpfully clarified that the energy for the second electron to enter the first shell is not 13eV, then what is it?

 

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Does it radiate the Balmer series of photons as a second electron falls into that first shell, and does anything else happen if it let's an electron go?

As I wrote before: It has different energy levels.
This also means different differences between energy levels. It will emit some radiation but the lines won't follow the Balmer series or any other series for neutral hydrogen.

What is the activation energy needed to prompt it to shed the electron?

That's something to look up in tables. Just 0.75 eV.

If hydrogen molecules and electrons are floating around in a vacuum with, say, 1eV electrons, do they tend to form H-, (H2)-, (H3)- and if all of the above, then what is the relative proportion of species?

That will depend on the properties of the hydrogen gas. That's also something I wrote before. Temperature, density, initial state if we don't talk about equilibrium, ... and also the electron density and energy distribution. You'll also need something to contain the total net charge if it is significant.

Presumably if (H2)- meets more electrons it might break up and form two H- ions?

That is a possible process if the electron energy is sufficient (you can calculate that). It doesn't necessarily happen.

And as you've now helpfully clarified that the energy for the second electron to enter the first shell is not 13eV, then what is it?

See above.

 

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As I wrote before: It has different energy levels.
This also means different differences between energy levels.

OK, fine, can you suggest a location for such tables? I cannot find anything listed for ionisation energies of negative ions.

Just 0.75 eV.

I get the impression that is 'just' one energy level and there aren't any more.

I'm really 'shocked' to discover at this point in my engineering life, if that is the right word, for hydrogen having a LOWER energy state than -13eV! I need some time to get over this! ;)

The reason I think that is that if you had a proton with one electron in the lowest shell and one electron in the second, it will not stay in the second shell and immediately fall straight to the inner shell, because surely the ion cannot be excited or it will lose that delicate extra electron?

I have to catch up with this new notion that that the negative ion has a LOWER ground state than the atom!

Could this also mean that a di-ionic (H2)2- ion has a lower ground state than the diatomic H2? You take two H-, which are lower ground state than two H atoms, then they might bond covalently to give this di-ionic (H2)2- ion?

That will depend on the properties of the hydrogen gas. That's also something I wrote before. Temperature, density, initial state if we don't talk about equilibrium, ... and also the electron density and energy distribution. You'll also need something to contain the total net charge if it is significant.That is a possible process if the electron energy is sufficient (you can calculate that). It doesn't necessarily happen.

I realize there may be lots of variables, but are there any interesting/common/astronomically interesting/industrially significant examples where there are a mix of hydrogen-molecular negative ions? If you have any references, theoretical/empirical formulae, etc., that would be good ... basically anything that can put a few physical numbers around such a behaviour?

 

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OK, fine, can you suggest a location for such tables?

I linked to one in my previous post... just at the place where I quoted the number I got from there.

I get the impression that is 'just' one energy level and there aren't any more.

It is the ground state. There are higher states, too.

The reason I think that is that if you had a proton with one electron in the lowest shell and one electron in the second, it will not stay in the second shell and immediately fall straight to the inner shell, because surely the ion cannot be excited or it will lose that delicate extra electron?

It can stay in the second shell for a while until it falls down. Just like a single electron can.

I have to catch up with this new notion that that the negative ion has a LOWER ground state than the atom!

That just means an atom can become a stable negative ion.

Could this also mean that a di-ionic (H2)2- ion has a lower ground state than the diatomic H2? You take two H-, which are lower ground state than two H atoms, then they might bond covalently to give this di-ionic (H2)2- ion?

I would be really surprised if that works. It should be unbound, the H- repel each other too much.

I realize there may be lots of variables, but are there any interesting/common/astronomically interesting/industrially significant examples where there are a mix of hydrogen-molecular negative ions? If you have any references, theoretical/empirical formulae, etc., that would be good ... basically anything that can put a few physical numbers around such a behaviour?

You don't get significant net charge naturally and I'm not aware of applications for it either.

 

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I linked to one in my previous post... just at the place where I quoted the number I got from there.

I asked for a table with the different energy levels in because you said there were different energy levels, and the one you linked to had one energy level for H, hence my confusion.

 

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You asked for "such tables" after you asked for the total energy and I discussed a table of this value for each element with one extra electron.

NIST might have spectral data for more states, you can calculate the energy level based on that.