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Ionization energy in a vaccum

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Andy

Wed Dec 11 2013, 05:15AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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Posts: 874

Hi all
Looking at

Al has 5.98eV for the first electron ionization. I was wondering if there is two Al electrodes inside a vacuum chamber, with 8 volt between the electrodes, would the vacuum pass the electrons from one electrode to the other.
If the vacuum is at what a basic vacuum pump can do, say 1psi spaced 1cm apart.
Or would the energies be of air instead of Al?

Thanks

vircator

Wed Dec 11 2013, 07:15AM

Registered Member #3217 Joined: Mon Sept 20 2010, 06:14AM
Location: Switzerland
Posts: 15

I am not a physicist or chemist but I think eV is an energy unit, Volt is a potential difference. Yes, the electron would get 8eV of kinetic energy IF it has passed that potential difference (eg flying from a cathode to an anode). My gut feeling says that the strength of the electrical field (Which also depends on the shape of the electrodes) is relevant and not only the potential difference.

...

Wed Dec 11 2013, 04:13PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

There are a few problems with your plan:
1. The 'first electron ionization' energy is in fact the energy required to ionize the first valence electron, however the only (practical) ways to couple that energy to an atom is to hit it with a photon of greater or equal energy, or heat the surface hot enough so that there is enough thermal energy to ionize (this is how filaments work in vacuum tubes). If you just apply a static field of 5v/cm to the atom nothing will happen, because the electric field due to the nucleus is like 10^12v/cm. However, if you were able to ionize an electron off the surface with a photon, you could add an additional 5ev of energy to it using your proposed setup.
2. If you could ionize an electron (say using a UV light source, a 250nm photon has roughly 6ev of energy), it will not travel very far with a 1psi backing pressure, the mean free path is ~1micrometer, so after a few micrometers all of the electrons will have crashed into something. For electron experiments you would want at least a decent vacuum pump, a normal 'rotary vane' type pump will get down to tens of mTorr, or ~.0001psi, which is still only a mean free path of a few mm, ideally you would go to something like a turbo or diffusion pump for dealing with low energy electrons.

Andy

Wed Dec 11 2013, 04:57PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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If I was to use a static field, would there be a rough guide to the voltage needed to emit a electron. How hot would the filament have to be at low voltages?,Does it matter how much watts to the UV filament, how close does it have to be to the electrodes? How much more would the mean free path be with hydrogen, I've seen a calculator on the net,

but it needs to know the angstroms value.

Sorry about the twenty questions :)

klugesmith

Wed Dec 11 2013, 07:12PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Andy, look up "field emission".
To get a cold metal surface to emit electrons, without illumination,
you want sharp needle points and many thousands of volts.
The resulting E field strength at the metal/vacuum interface is on the order of 10^9 volts/meter.
Compare that to breakdown of air at normal pressure & temperature: 3 x 10^6 volts/meter.

Bored Chemist

Wed Dec 11 2013, 08:55PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I'm fairly sure the 5.98 eV figure is for an aluminium atom in the vapour phase.

If you are thinking of knocking electrons out of metal with UV you need to consider this parameter instead.

The good news is that it's only about 4 eV. The bad news is that you need a clean aluminium surface to get that value and a clean surface needs an ultra high vacuum to maintain it for any useful length of time.

Andy

Thu Dec 12 2013, 05:40PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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Posts: 874

Hi klugesmith
I'm not yet sure, but I don't think I want a arc, just the low powered electron source, that won't make a conductive path.

Hi Bored Chemist
What metal would need the least amount of cleaning and a rotary pump, I'm assuming the Al will oxides, tungsten/titanium? I should be able to get a sanitizer UV lamp, but would a xray tube have more effect.

For the UV light, what would be a good plastic that would let the light through, in the ranges of 315 â€“ 280(3.94 â€“ 4.43 eV) nm,280 â€“ 100 nm(4.43 â€“ 12.4 eV),200 â€“ 122 nm(6.20 â€“ 10.16 eV)

The Ultraviolet C and Mg electrode might work, would Mg need cleaning?Hydrogen atmosphere?
W 4.32 â€“ 5.22eV
Ti 4.33
Cu 4.53 â€“ 5.10
Mg 3.66
Zn 3.63 â€“ 4.9
Fe 4.67 â€“ 4.81

Thanks

Bored Chemist

Thu Dec 12 2013, 08:57PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I wasn't kidding about UHV

And most plastics are not good at transmitting UV- the manufacturers often add materials which absorb UV to prevent the light degrading the plastic.
Polymethyl methacrylate (plexiglass or perspex) isn't too bad.

Andy

Thu Dec 12 2013, 09:16PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

I was hoping for 1 year....But is in the seconds,Is that with low voltages?
A needle point 0.5mm2 or less and about >1kv could be used, with hopeful the back scattered rays continually ionize the anode. Just need a kick start, I think

If the voltage was AC would it still get coated with the gases positive ion?

For hydrogen the angstroms is 0.25, at 0.5psi gives 4mm which should be enough from that calc.

Proud Mary

Fri Dec 13 2013, 04:40PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

In gaps narrower than 10 Î¼m, current in the Î¼A regime can flow before breakdown occurs in air at atmospheric pressure, due to ion-enhanced field emission from the cathode.

Paschen's Law goes off the rails on the microscale, which is why this is such a cool new subject.

It's all explained here:

How can I make gaps less than 10 Î¼m with the basic tools of the home workshop, that's what I want to know.

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