The laser-field-modified dipole response of the first ionization threshold of helium is studied by means of attosecond transient absorption spectroscopy. We resolve light-induced time-dependent structures in the photoabsorption spectrum both below and above the ionization threshold. By comparing the measured results to a quantum-dynamical model, we isolate the contributions of the unbound electron to these structures. They originate from light-induced couplings of near-threshold bound and continuum states and light-induced energy shifts of the free electron. The ponderomotive energy, at low laser intensities, is identified as a good approximation for the perturbed continuum response.

中文翻译：

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连续阈值的亚秒瞬态吸收

通过阿秒瞬态吸收光谱研究了氦的第一电离阈值的激光场修饰偶极响应。我们解析了光吸收光谱中低于和高于电离阈值的光诱导时间依赖性结构。通过将测量结果与量子动力学模型进行比较，我们分离了未结合电子对这些结构的贡献。它们源自光诱导的接近阈值的束缚态和连续态以及光诱导的自由电子能量耦合。在低激光强度下，质子能被认为是扰动连续谱响应的良好近似。