Figure 5. Tunnel and above-barrier ionisation of He atoms.

(a) For an electric-field strength of lower than 104.9 V/nm, electrons (blue circles) from the ground state can leave the He atom via tunnel ionisation through the potential barrier (pink). The remaining electron of the He⁺ ion will then be much more tightly bound. (b) For higher electric-field strengths, the electrons will be freed above the barrier. After ionisation, the electrons will initially have the position x_i and the velocity v_i.