Strong-field manipulation of autoionizing states is a crucial aspect of electronic quantum control. Recent measurements of the attosecond transient absorption spectrum (ATAS) of helium dressed by a few-cycle visible pulse [C. Ott et al., Nature (London) 516, 374 (2014)] provide evidence of the inversion of Fano profiles. With the support of accurate ab initio calculations that reproduce the results of the latter experiment, here we investigate the new physics that arise from ATAS when the laser intensity is increased. In particular, we show that (i) previously unnoticed signatures of the dark 2p21S doubly excited state are observed in the experimental spectrum, (ii) inversion of Fano profiles is predicted to be periodic in the laser intensity, and (iii) the ac Stark shift of the higher terms in the sp+2,n autoionizing series exceeds the ponderomotive energy, which is the result of a genuine two-electron contribution to the polarization of the excited atom.
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机译：自电离态的强场操纵是电子量子控制的重要方面。最近的几个周期可见脉冲覆盖的氦的阿秒瞬态吸收光谱（ATAS）的测量[C. Ott等人，Nature（London）516，374（2014）]提供了Fano剖面反演的证据。借助精确的从头算起的计算来再现后一个实验的结果，在这里，我们研究了当激光强度增加时，ATAS产生的新物理学。特别是，我们表明（i）在实验光谱中观察到了黑暗的2p21S双激发态以前未曾注意到的特征，（ii）预测Fano轮廓的反转在激光强度上是周期性的，并且（iii）ac Stark sp + 2，n自电离序列中较高项的位移超过了质子能，这是由真正的双电子对激发原子的极化造成的。
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