Photoelectron initial conditions for tunneling ionization in a linearly polarized laser

A precise measurement of the initial kinetic energy of photoelectrons born in a high-intensity, linearly polarized laser has been made in the long-pulse tunneling limit. The shape of the angular distribution of electrons outside the laser focus is related to the initial momenta the electrons gain from the field. The measurements of nonzero drift momenta for high-charge states of neon indicate tunneling ionization is occurring and agree with the predictions of the Ammosov-Delone-Krainov (ADK) model [M. V. Ammosov, N. B. Delone, and V. P. Krainov, Sov. Phys. JETP 64, 1191 (1986)]. The validity of this model allows the value of the electron's initial kinetic energy to be limited to approximately 0.2% of its average quiver energy.