[pfdietz]

And, in fact, you WANT heavy ions, as the thrust/area of an ion engine at a given exhaust velocity scales as the square of the ion mass/charge ratio. The thrust in an ion engine is limited by space charge (where the charge of the ions between the accelerating grids becomes similar to the charge on the grids). Using heavier ions also reduces the ionization energy/mass. There's been work on using molecules or small droplets ("colloidal thrusters") to get even higher mass/charge, but you need to totally avoid generation of fragments with low mass/charge as they will dominate the current.