Water dissociation constant $K_w$ in vapour, or are there charges entities ($\ce{H3O+}$, $\ce{HO-}$,...) in water vapour?

When water is in vapour state, to which degree is there still some dissociation with the existence of charged species like $\ce{H3O+}$ and $\ce{HO-}$? (I would be interested by any reference to experimental studies that attempted to measure the amount of such charged species in water vapour.)


Alternatively, would you know what experimental techniques could be used to make a direct experimental measurement of the amount of charges species in a gas?


