We study the mechanism of circular dichroism (CD) in $ {{\rm He}^ + } $ under XUV and NIR pulses in the experimental condition presented by Ilchen et al. [Phys. Rev. Lett. 118, 013002 (2017) [CrossRef] ] 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 $ {{\rm 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.

中文翻译：

---

He ⁺离子的圆二色性研究

我们在Ilchen等人提出的实验条件下研究了XUV和NIR脉冲下$ {{\ rm He} ^ +} $中圆二色性（CD）的机理。[物理 莱特牧师 118，013002（2017）[交叉引用] ]并通过求解在动量空间与时间相关的薛定谔方程（TDSE）的范围内的脉冲的强度和持续时间的。该方法能够研究$ {{\ rm He} ^ +} $的电离和中间跃迁几率详细地。我们显示最低阶摄动理论（LOPT）在质量上足以描述实验结果，并且发现电离有利于在LOPT体制内外的NIR强度同时旋转和反向旋转。在实验脉冲持续时间中，我们表明，由于来自激发态的单光子吸收，在反向旋转情况下，沿着第一个阈值以上电离峰存在一个亚峰，而这不能通过求解短时程TDSE来获得。 22 fs，因为能带宽度太宽而无法解决该子结构。我们介绍了CD值随NIR强度的变化。