(a) Because of collisions and recombination with other molecules in the gas, the ions in the gas have no chance of reaching their corresponding electrons at atmospheric pressure. Ions have a chance to reach their corresponding electrons at low pressures, resulting in current flow.
(b) The work function is the minimal energy required for an electron in the conduction band to leave the metal. Because these electrons occupy distinct energy levels, they emit electrons with varying energies for the same incident radiation.