Attosecond-resolution two-electron harmonic emission

Highlights: • A two-electron time-dependent Schrödinger equation analysis. • Harmonic emission spectrum from the neutral He atom in intense laser field. • An extended harmonic plateau with many new harmonic cutoffs. • A propose model of double recombination arising from electron correlation effect. • A general characteristic of double recombination in the two-electron system. - Abstract: Two-electron harmonic emission from the He atom has been investigated by solving the two-electron time-dependent Schrödinger equation (TDSE), which exhibits an extended plateau and many new harmonic cutoffs beyond the classical single-electron harmonic cutoff. Theoretical analyses show that these extended new cutoffs are caused by the sequential double recombination and the nonsequential double recombination of the two electrons, which is a general characteristic for the two-electron harmonic emission and which is revealed through the investigation on laser parameter effects. Moreover, from analyzing the time-dependent wave functions, the motions of the two electrons and the single and double ionization time have been described