Recoil-Ion Momentum Distributions for Single and Double Ionization of Helium in Strong Laser Fields

We have measured the momentum distributions of singly and doubly charged helium ions created in the focus of 220 fs, 800 nm laser pulses at intensities of (2.9--6.6)×10¹⁴W/cm². All ions are emitted strongly aligned along the direction of polarization of the light. We find the typical momenta of the He²⁺ ions to be 5--10 times larger than those of the He¹⁺ ions and a two peak structure at the highest intensity.