Non-sequential double ionization of H2 using ultrashort 10 fs laser pulses

Non-sequential double ionization of H-2 is clearly observed using 10 fs laser pulses in the 10(13)-10(14) W cm(-2) intensity range using proton spectra from the H++H+ dissociation channel. Proton spectra recorded using linear and circular polarizations allow one to identify rescattering of the first ionized electron as the major effect responsible for non-sequential double ionization. At intensities below 10(14) W cm(-2), double ionization is dominated by recollision excitation of H-2(+) followed by field ionization of transient excited states. Comparisons with spectra recorded at 40 fs show that sequential double ionization paths and the related charge resonance enhanced ionization are severely reduced at 10 fs and low laser intensities because H-2(+) does not have enough time to stretch up to internuclear distances where ionization rates are very high.