This has been discussed ad nauseam before here. Once a free electron gets loose in an electric field, it will be accelerated and crash into the gas atoms. If the electron's KE is above a level sufficient to cause the ionisation of that gas molecule, it will ionise liberating more electrons each of which then do their own accelerating and ionising. Depending on the density and electric field gradient, this process will either cascade or will be suppressed.Justin Nichols wrote:But where did the first ionized atoms come from?
Where does that first loose electron come from? Doesn't matter, and you'd never be able to find out. Background radiation, a field emission process at a sharp point, thermionic emission, just a fluctuating background electric field? Doesn't matter. Point is, you'll usually find a loose electron in a mass of gas and it only takes one to begin the cascade.