The double ionization of helium in the strong-field limit has been studied using an electron-ion coincidence technique. The observed double ionization electron energy spectra differ significantly from the single ionization distributions. This gives new support to the rescattering model of double ionization and explicitly reveals the role of backward electron emission following the e-2e ionizing collision.

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High-order harmonics have been generated by the use of an annular laser beam. The nonlinearity of harmonic production and the shorter wavelengths involved cause the harmonics to emerge strongly peaked on the laser axis. Thus the harmonics emerge from the focus inside the missing portion of the laser beam. This permits the laser to be blocked by an aperture that passes the harmonics.