Molecular propellant in ion engines

Question

This question was prompted by discussions on this question. Can there be an advantage to using a molecular propellant in an ion engine, as opposed to noble gasses or iodine? (Although iodine actually does look pretty cool.) There's a large variety of molecules to choose from, but it seems like you could find some that are cheap, heavy, easy to ionize, and store well.

I can think of a disadvantage, which is dissociation. Like the quench gas in a GM tube, except quench gasses are specifically chosen for that property.

Answer 1

I have found a little bit of an answer. One possibility is carbon-60, or "buckyballs". https://www.jpl.nasa.gov/news/news.php?feature=5698 But that article is from 1991, so if it hasn't caught on by now, it probably won't.

Another alternative is air--whatever happens to be in it. That is being developed by the ESA. But for the availability of it, not for advantageous material properties. https://futurism.com/esa-ion-thruster-breathes-air

In general, when they talk about alternatives to xenon, they usually mean krypton, argon, iodine, or sometimes hydrogen. Molecular propellants don't seem to be a hot topic. But it seems worth looking at.