摘要
We study the mechanism of circular dichroism (CD) in He + under XUV and NIR pulses in the experimental condition presented by Ilchen et al. [Phys. Rev. Lett. 118, 013002 (2017)] and in a range of pulse intensities and durations by solving the time-dependent Schrödinger equation (TDSE) in momentum space. The method enables investigation of the ionization and intermediate transition probabilities of He + in detail. We show that the lowest-order perturbation theory (LOPT) is qualitatively sufficient to describe experimental results and find that ionization favors co- and counter-rotating pulses for NIR intensities in and beyond LOPT regimes, respectively. In the experimental pulse duration, we show that a sub-peak along the first above-threshold ionization peak exists in the counter-rotating case due to one-photon absorption from excited states, which cannot be obtained by solving TDSE with a short duration of 22 fs, since the energy band width is too wide to resolve this substructure. We present the variation of CD values with NIR intensities.