Hydronium

So the Hydronium ion, the representation H3O+ of the free (solvated) protons in an aqueous acid, is an interesting concept. In 1983, a paper published in Physical Review Letters shows evidence of the free ion in the gas phase.
While the details of the paper are quite complex, it's interesting to see that the reaction of H2 and O2 will proceed through the H3O+ ion and is observable on short time scales.
Free protons!