Encoded in the transient x-ray absorption (XAS) and magnetic circular (MCD) response functions resides a wealth of information of the microscopic processes of ultrafast demagnetization. Employing state-of-the-art first-principles dynamical simulations we show that the experimentally observed energy shift of the $L_3$ XAS peak in Ni, and the absence of a corresponding shift in the dichroic MCD response, can be explained in terms of laser-induced changes in band occupation. Strikingly, we predict that for the same ultrashort pump pulse applied to Co the opposite effect will occur: a substantial shift upward in energy of the MCD peaks will be accompanied by very small change in the position of XAS peaks, a fact we relate to the reduced $d$-band filling of Co that allows a greater energetic range above the Fermi energy into which charge can be excited. We also carefully elucidate the dependence of this effect on pump pulse parameters. These findings (i) establish an electronic origin for early-time peak shifts in transient XAS and MCD spectroscopy and (ii) illustrate the rich information that may be extracted from transient response functions of the underlying dynamical system.

中文翻译：

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X 射线吸收峰位移的电子起源

在瞬态 X 射线吸收 (XAS) 和磁圆 (MCD) 响应函数中编码包含大量超快退磁微观过程的信息。采用最先进的第一原理动力学模拟，我们表明实验观察到的能量转移${\mathrm{大号}}_3$Ni 中的 XAS 峰，以及二向色性 MCD 响应中没有相应的变化，可以用激光引起的带占据变化来解释。引人注目的是，我们预测对于施加到 Co 的相同超短泵浦脉冲将发生相反的效果：MCD 峰的能量大幅向上移动将伴随 XAS 峰位置的非常小的变化，我们与减少$d$Co 的能带填充，允许在费米能以上的更大能量范围内可以激发电荷。我们还仔细阐明了这种影响对泵脉冲参数的依赖性。这些发现 (i) 为瞬态 XAS 和 MCD 光谱中的早期峰移建立了电子起源，并且 (ii) 说明了可以从潜在动力系统的瞬态响应函数中提取的丰富信息。