Measurement of the Initial Condition of Electrons Ionized by a Linearly Polarized, High-Intensity Laser

We have measured the angular spectrum of electrons born in a linearly polarized, high-intensity laser focus. The spectrum is directly related to the initial conditions of the ionized electrons in the plane perpendicular to the propagation direction. Our measurements for high charge states of neon are in good agreement with predictions of quasistatic tunneling models and limit the initial electron kinetic energy to approximately 0.5% of its average quiver energy. This measurement is important for studies of high-order harmonic generation and direct double ionization.