Stages of Gas Thermal Ionization

[gareth]

I'm trying to get my head around how a gas is thermally ionised.

I understand you can ionize a gas by irradiating it with some short wave radiation to overcome the work function of the atom, right?

But I also know you can thermally ionize a gas, i.e. the collisions between the particles themselves actually knock each others electrons out of orbit, is this right?

How does this come about?

thnx

 

[Oberst Villa]

As far as I know, both mechanisms you describe are correct. I think there is even one more "exotic" mechanism where an atom is ionized by a very strong electric field.

As for the "how", the only thing I know is that there has to be "something" that provides the minimum required energy (i.e. the ionization energy). If more energy is supplied, the difference is transformed into kinetic energy of the liberated electron.

 

[gareth]

thanks for the reply,

I imagine that the Coulomb repulsion of the electrons when they get in very close proximity causes the ionisation. This may occur in a hot gas where the atoms are colliding at high speeds.

But I'm still not sure about this mechanism, what actually happens during the collision?

 

[cadnr]

During the collision, kinetic energy of the moving atoms is converted into excitation energy of an electron. if the excitation energy is above the threshold for ionisation, then the electron may be lost from the atom.

You'd be right in thinking the coulomb interaction is responsible for transferring the energy.

 

[gareth]

So basically, if the atoms are traveling fast enough when they collide then an electron can be emitted.

Is there a formula that could calculate the expected degree (%) of ionisation in a particular gas (monatomic for simplicity), for a given temperature?

 

[cadnr]

Um, you might look up the Saha equation... that could be relevant

 

[gareth]

just what i needed, thanks cadnr

 

[cadnr]

During the collision, kinetic energy of the moving atoms is converted into excitation energy of an electron. if the excitation energy is above the threshold for ionisation, then the electron may be lost from the atom.

You'd be right in thinking the coulomb interaction is responsible for transferring the energy.

I suppose, however, that for sufficiently high energy collisions, the Pauli exclusion force might also come into play

 

[gareth]

I suppose, however, that for sufficiently high energy collisions, the Pauli exclusion force might also come into play

This is interesting, the principle comes into play when there is an 'overlap' in the deBroglie wavelengths, which is the case is a solid giving the 'band' nature of the solid energy levels.

But if we have a very hot gas, and two atoms are involved in a collision at very high speeds, the electrons might get so close as to incorporate the exclusion principle?

So what would the effect be to the gas/plamsa?

I assume the electrons would have obey the principle and fit into energy bands accordingly, making a degenerate type gas as in a solid.

Any thoughts?

 

[gareth]

Another question, this time about the Saha equation which you mentioned cadnr.

I've had a good look at a few different versions of it. It seems you need to know the partition function of the element in question if you want to evaluate it properly.

Does anyone know where I can find these?

 

[cadnr]

Here are some tabulated rotational partition functions for various molecules: http://www.astro.uni-koeln.de/site/vorhersagen/catalog/partition_function.html

The values are listed as log10(Q(T)) where Q(T) is the partition function at temperature T.

 

[gareth]

Thanks for the link,

what I would like though is partition functions for monatomic, single element gasses.

I know you can calculate these using the partition funtions, but all the examples I've seen deal with H has, which is obviously a pretty simple solution,

the degeneracy goes something like g=2n^2, n being the number of states available to the electron, so in H the degeneracy terms for the H atom look something like;

g_ion = 1 (because there is no degeneracy with a single proton)

g_neutral = 2 (because n=1, one state available)

My problem is claculating these for more complex systems, say if we have a more complex gas like N for example, how would the degeneracy terms look there? (I know we have to consider the temperature effect on the number of levels, but that is usually negligable and I'll look into that later)


Thanks