Left panel: Measured proton spectra recorded with 10 fs laser pulses and linear and circular polarizations. The dashed curve represents the expected spectrum from an instantaneous double ionization of H2. Right panels: Two-electron probability distribution at different times and internuclear distances. In the top right panel, electrons are emitted in the same direction due to rescattering and field induced double ionization. In the bottom right panel, electrons are emitted in opposite directions due to rescattering and auto-double ionization
At intensities below 1014 Wcm-2, double ionization of H2 is dominated by recollision excitation of H2+ followed by field ionization of transient excited states. A full quantum non-Born-Oppenheimer model shows that rescattering produces a coherent superposition of excited states which present a pronounced transient H+H- character. This excitation is followed by either field-induced double ionization or the formation of short-lived attosecond autoionizing states. In the first case, electrons are ejected in the same direction. In the second case, electrons are ejected in opposite directions. See the butterfly pattern in the following figure.
At higher intensities above 2x1014 Wcm-2, the dynamics is dominated by the sequential ejection of electrons. The proton spectra are shifted to higher energies as the pulse duration is reduced down to 10 fs. This is an important difference with non sequential double ionization which is basically a sub-cycle process.
The good agreement between experiments and theory down to 10 fs makes it reliable to theoretically demonstrate the very fast nuclear dynamics of H2 for pulse durations as short as 1 fs and the significant role of the carrier-envelope phase offset for durations shorter than 4 fs. Finally, in a few-cycle laser pulse, Charge Resonance Enhanced Ionization (discovered by A. Bandrauk et al.) does not contribute anymore to the double ionization of H2, since the H2+ ion does not have enough time to stretch up to the so-called critical distance. Using a pump-probe excitation scheme, we have shown that charge resonance enhanced ionization is a very sensitive probe for the detection of pre- and post-pulses.
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