Accurate measurement of tunneling ionization rates of atoms in a high-intensity laser field

Tunneling ionization of noble gas atoms is important for understanding the physics of strongly driven atoms and for developing new technologies such as ultrafast coherent x-ray sources and laser-based electron accelerators. Measurements of tunneling rates for atoms ionized by a high-intensity laser pulse have achieved a factor-of-ten improvement in accuracy over previous experiments, providing unprecedented detail about high-intensity laser-atom interactions.; The measurements employ a circularly polarized, 2-ps, 1053-nm laser pulse with a peak intensity of 1.5 x 1017 W/cm2 to ionize low charge states of helium and neon. Under these conditions, electrons produced during ionization are ponderomotively scattered out of the laser focus and gain an amount of energy proportional to the intensity at the moment of ionization. The measured electron energy (0--4 keV) is insensitive to variations in the peak laser intensity, allowing the ionizing electric fields to be determined with an accuracy better than 2%. Detailed Monte Carlo simulations of electron motion in the focus, which calibrate the ionizing fields to the electron energies, confirm the accuracy of the measurements.; The inferred ionization rates provide rigorous tests of available tunneling theories. The helium data show excellent agreement with the semiclassical theory of Ammosov, Delone, and Krainov (ADK). The results for hydrogenic helium (He1+), in particular, agree well with analytic and numerical solutions of the Schrodinger equation. The measurements verify both the importance of the long-range nature of the Coulomb potential and the effect of multielectron interactions in determining the ionization rate. The neon data show a small but significant breakdown of the ADK theory, which is attributed to polarization of the atom by the laser field prior to ionization. Atom polarization may imply that tunneling ionization intensities are described by a universal scaling relation. The neon data also provide tentative evidence for the DC Stark shift of the atomic ground state and its effect on tunneling ionization rates.