# Is a plasma necessarily made of monoatomic ions?

Is it possible to have a plasma made of polyatomic ions instead of monoatomic ions?

I want to know all the details why such a thing may be attainable or not and, if possible, what methods we can use to create such a substance.

• Good question. The electrons that form the molecular bonds are those which are most weakly bound in the first place. That's why they're orbiting between two atoms, instead of orbiting just a single atom. And those weakly bound electrons would probably be the first to affected in ionization. Either they're stripped off, or they pair up with a donated electron instead. Either way, they no longer pair up to form a molecular bond. This simple theory does predict that molecules with a double bond are more likely to survive ionization. An O2- ionized molecule still has one bond left. – MSalters Sep 9 '14 at 15:34

Yes, a plasma contains positive ions and negative electrons, but the positive ions don't need to be atoms. Actually the negative charges don't even need to be electrons. For example, plasma etching of silcon is done using a sulphur hexafluoride plasma that consists of (mainly) SF$_5^+$ and F$^-$ ions.

I'm not sure that the word plasma has a precise definition, other than vaguely meaning a gaseous state containing charged particles but electrically neutral overall.

• Doesn't hex in hexafloride mean 6? – Kyle Kanos Sep 9 '14 at 15:06
• @KyleKanos: SF$_6 \rightarrow$ SF$_5^+ +$ F$^-$ – John Rennie Sep 9 '14 at 15:07
• Plasma is pretty well defined. I'd also change 'charged' because plasmas are often net neutral. – user121330 Sep 9 '14 at 19:08
• Better definition which also makes clear that plasmas should not be called 'charged'. – user121330 Sep 9 '14 at 19:18
• @JohnRennie suggest you include some information about HOW such plasmas are created, as that was part of the OPs question. I see this too often on this site - only part of the question is answered yet accepted - I don't understand. I also agree with user121330 that a plasma is well-defined (I'm speaking as one trained in plasmas). – user3814483 Sep 10 '14 at 12:32

A plasma can be comprised of molecules so long as the temperature of the plasma is sufficient to ionize the molecule without dissociating it. Note that most definitions of plasmas do not really require a percentage of ionization (e.g., a gas that has only 1% of it ionized is considered a "cold" plasma whereas something much more than this is usually considered a "hot" plasma).

You can create a molecular plasma the same way as making a monatomic plasma: by heating the gas (obviously not to the point that the aforementioned disassociation occurs.)

• How can we ionize a molecule without dissociating it first? Are usually polyatomic ions created through that process? – 21Brunoh Sep 9 '14 at 14:59
• As I state, it would require the molecule to have the particular character of a lower ionization potential than dissociation potential. I am not an experimentalist, so I cannot say how polyatomic ions are generated, just that heating it should work. – Kyle Kanos Sep 9 '14 at 15:10

First, your typical neutral plasmas contain both ions and electrons. The ions may be polyatomic or monoatomic. There also exist non neutral plasmas, which are composed of only ions or electrons.

Plasmas in the laboratory are typically (though not necessarily) created by first injecting a gas into a vacuum chamber and then first ionizing that gas using large electric fields. The electric fields that initially break down the gas can come from different sources (e.g., toroidal electric fields in a tokamak, or large displacement fields in a capacitively-coupled antenna). Once the gas has been broken down to some extent, the plasma is typically sustained by electron-impact ionization.

Depending on the temperature of the electrons, and the composition of the initial gas (diatomic), one may either produce monoatomic or polyatomic ions. If the electrons are hot enough, it will be difficult to sustain a polyatomic ion species.

Plasma is essentially ANY ionized gas that has been electrified with extra electrons in both negative and positive states. If you were able to create an Oxygen plasma with polyatomic ions, it would still be a plasma. I would try to mix a second gas with Oxygen if I was attempting to do this. Here is my source: Plasma Treatment Basics