Two-photon double ionization of helium by temporally asymmetric attosecond pulses

Two-photon double ionization of helium induced by temporally asymmetric attosecond pulses is investigated. For asymmetric attosecond pulses with longer duration, compared to the π/4 orientation angle for elliptical energy distributions under the symmetric pulses, both the rising and falling edges of the attosecond pulse induce elliptical energy distributions to rotate π/8 rad around their centers, though in opposite directions. By using the time-dependent perturbation theory, it is found that these inclined elliptical energy distributions reflect that the emission of the two electrons is correlated according to the Pearson correlation coeffcient, and the final orientation of energy distribution originates from the contributions of three different pathways. For asymmetric attosecond pulses with shorter duration, compared to the elliptical energy distribution under symmetric pulses, the rising edge induces the two-electron energy distribution away from equal energy sharing, while the falling edge induces two-electron energy distribution toward equal energy sharing. These results may provide new insights into the dynamics of multi-electron processes driven by attosecond pulses.