Direct ionization as a result of photoionization- What is the probability?

[aquila14]

I'm working on a personal project trying to figure out if lasers can be effectively used to generate plasma from a gas at specific rates.

As part of this, I assumed that most of the ionization would take place because of the laser photons rather than the power generated by the laser. Working with this assumption, I'm now trying to figure out if there can be any direct ionization or stepwise ionization (from electron/ion impact) as a result of the photoionization. If so, how do you predict the percentage of each type of ionization?I've tried to look through a lot of textbooks and other sources, but haven't been able to find photoionization in combination with direct and/or stepwise ionization. This is my last ditch effort. I'll have to go with other assumptions if this theory doesn't pan out.

Please help!

Note: So far I'm haven't considered ionization equilibrium, so recombination doesn't factor in yet.

 

[SpectraCat]

I'm working on a personal project trying to figure out if lasers can be effectively used to generate plasma from a gas at specific rates.

As part of this, I assumed that most of the ionization would take place because of the laser photons rather than the power generated by the laser. Working with this assumption, I'm now trying to figure out if there can be any direct ionization or stepwise ionization (from electron/ion impact) as a result of the photoionization. If so, how do you predict the percentage of each type of ionization?


I've tried to look through a lot of textbooks and other sources, but haven't been able to find photoionization in combination with direct and/or stepwise ionization. This is my last ditch effort. I'll have to go with other assumptions if this theory doesn't pan out.

Please help!

Note: So far I'm haven't considered ionization equilibrium, so recombination doesn't factor in yet.

I don't really understand the types of ionization that you are drawing distinctions between in your question, so it's hard to tell where you are coming from with your question.

In general, the most efficient type of photoionization will be single-photon ionization .. i.e. the electron goes from bound to free after absorption of a single photon. The problem with that is that such transitions typically require light from the deep or vacuum UV, which complicates the laser source issue.

If you have a pulsed laser with fairly high power, and you focus it very tightly, you can get the power density high enough that you can ionize gases (i.e. air) non-resonantly. Basically the electric fields from the pulse gets large enough to rip electrons right out of the molecules. If you do this in air, you can get a tiny plasma with dimensions comparable to the focused laser spot that lasts for some fraction of the pulse duration (say a few picoseconds).

Other than that, you need to do some kind of coherent multi-photon ionization process. The intensities of these processes are typically very low unless you do REMPI (resonance enhanced multi photon ionization), but that usually requires at least one tunable laser. Even then, most REMPI processes do not have kind of efficiencies that would be needed to create and sustain plasmas.

My guess is about the best you can hope for is to achieve laser ignition of the plasma somehow, and then sustain it using RF fields and perhaps some kind of magnetic containment.

 

[aquila14]

Thank you spectracat. Your input helped! Now I know the direction I have to take. Thanks again, you cleared a lot of confusion in my head!