Voltage and ionization

[Jacques]

The ionization energy of H is 13.6 eV

With that information how can I estimate the volage I need to apply to 2 electrode in a H gas in order to get H+ ions ?

What is happening to the electrons ?

Can I get 100% of the H atom ionized?

Thanks

[swansont]

I don't do this sort of stuff, so this is a bit of guesswork; if we think classically for a moment, the electron and proton are separated by about .529 nm, and we have a 13.6 V potential difference, which is a field of 2.57e8 V/cm that you must overcome. I see numbers in various articles for field ionization of 1e7 - 1e8 V/cm for heavier atoms where (again classically) you'd have a larger separation of the electron and screening. So that's probably in the ballpark. Such large fields are often generated with intense lasers rather than a capacitor-type configuration.

 

Some applications use a surface with a large work function to get you part of the way there, so that in the presence of the external field you can get surface ionization.

 

edit: Ha! "Field ionization of hydrogen occurs at approximately 250 MV/cm" so my little calculation was right!

http://www.archive.org/stream/encyclopedicdict017808mbp/encyclopedicdict017808mbp_djvu.txt

Edited May 28, 2008 by swansont
found corroboration

[Jacques]

Thanks Swansont

That was the way I was thinking of calculating it, but I know that in neon sign they use voltage in the kV range and the distance between the electrode can be several meters.

Neon ionization energy is 21.6 eV... My guess is that it is not field ionization that occur in a neon sign.

 

Here is a link where I found lot of information about ionization mass-spectrometry

Click on ion source in the left pane

Edited May 28, 2008 by Jacques
multiple post merged

[swansont]

Thanks Swansont

That was the way I was thinking of calculating it, but I know that in neon sign they use voltage in the kV range and the distance between the electrode can be several meters.

Neon ionization energy is 21.6 eV... My guess is that it is not field ionization that occur in a neon sign.

 

Here is a link where I found lot of information about ionization mass-spectrometry

Click on ion source in the left pane

 

No, it's not. If you have a hot cathode, you "boil" off electrons and accelerate them, and once they have KE > the ionization energy, they can strike an atom and ionize it (or possibly excite it with less energy) and you get a cascading effect, and the recombination results in emitted light.

 

Neon lights are probably cold-cathode, though. You can get surface ionization, possibly assisted from the field, of a coating on the cathode, and this gives you the electrons that cause ionization. And/or you put in some other gas with a lower ionization potential to help get things started. Also, the material on the cathode can be a material that gives off secondary electrons, like in a photomultiplier. And thermal collisions between atoms can give you the occasional ionization, which will act as a seed for the cascade once you turn the voltage on.

[Jacques]

Thanks again!

I asking these questions because I want to know the best method to get H+ ion.

What I mean by best is :

The most efficent (the portion of the energy that is put in the system, that is used to create ion)

The purest ion (ion without the electron being around ready to recombine)

The bigest volume ( being able to produce a lot of ion)

The most mono-energitic (ion having a small energy distribution)

 

Maybe I should have ask the question directly, but I needed some understanding of the ionization process.

[swansont]

Accelerating electrons, like in a discharge tube, to impinge on the atoms will do a pretty good job, as long as you have the right pressure. Large enough for there to be targets to hit, but small enough you can get a current to flow.