Why is it important to be a Nobel gas?
2) won’t corrode your thruster as much.
3) Chemical and fire safety aren’t concerns.
4) Is a fluid already so doesn’t need to be heated to flow.
Also, being monotomic makes it easier to model everything. A more complex molecule could have weird chemistry that ends up producing solid particles that impinge or maybe condense on your spacecraft (coating lenses, etc).
http://aerospace.mtu.edu/__reports/Conference_Proceedings/20...
EDIT: and iodine, ie the subject of this study, is also significantly better than Xenon in this metric: “ A possible alternative is iodine, which is much more abundant and cheaper than xenon and can be stored unpressurized as a solid. In addition, both atomic and diatomic iodine have a lower ionization threshold, and diatomic iodine has a relative mass that is almost twice that of xenon.”
So I’m imagining a solid block of iodine (it is in cold space), and when the engine needs thrust, some current from the solar cells is passed through a heating element to drive sublimation + lots of complex rocket science that I do not know of.
If you are referring to the noble gases, I don’t think they are cooled to the point of solids; just high pressure liquids..
You can technically liquefy/boil iodine (I've done it a few times). It happens around 110C IIRC.