CNR-INFM, National Laboratory for Ultrafast and Ultraintense Optical Science, Department of Physics, Politecnico of Milan, Piazza L. da Vinci 32, 20133 Milano, Italy.
Nature. 2010 Jun 10;465(7299):763-6. doi: 10.1038/nature09084.
For the past several decades, we have been able to directly probe the motion of atoms that is associated with chemical transformations and which occurs on the femtosecond (10(-15)-s) timescale. However, studying the inner workings of atoms and molecules on the electronic timescale has become possible only with the recent development of isolated attosecond (10(-18)-s) laser pulses. Such pulses have been used to investigate atomic photoexcitation and photoionization and electron dynamics in solids, and in molecules could help explore the prompt charge redistribution and localization that accompany photoexcitation processes. In recent work, the dissociative ionization of H(2) and D(2) was monitored on femtosecond timescales and controlled using few-cycle near-infrared laser pulses. Here we report a molecular attosecond pump-probe experiment based on that work: H(2) and D(2) are dissociatively ionized by a sequence comprising an isolated attosecond ultraviolet pulse and an intense few-cycle infrared pulse, and a localization of the electronic charge distribution within the molecule is measured that depends-with attosecond time resolution-on the delay between the pump and probe pulses. The localization occurs by means of two mechanisms, where the infrared laser influences the photoionization or the dissociation of the molecular ion. In the first case, charge localization arises from quantum mechanical interference involving autoionizing states and the laser-altered wavefunction of the departing electron. In the second case, charge localization arises owing to laser-driven population transfer between different electronic states of the molecular ion. These results establish attosecond pump-probe strategies as a powerful tool for investigating the complex molecular dynamics that result from the coupling between electronic and nuclear motions beyond the usual Born-Oppenheimer approximation.
在过去的几十年中，我们已经能够直接探测与化学转化相关的原子运动，其发生在飞秒（10^(-15)秒）时间尺度上。然而，只有最近发展起来的孤立阿秒（10^(-18)秒）激光脉冲才能使我们研究原子和分子在电子时间尺度上的内部工作。这种脉冲已被用于研究原子光激发和光离化以及固体中的电子动力学，并且在分子中可以帮助探索伴随光激发过程的快速电荷重新分布和局域化。在最近的工作中，使用飞秒时间尺度监测了 H(2)和 D(2)的离解电离，并使用少周期近红外激光脉冲对其进行了控制。在这里，我们报告了一个基于该工作的分子阿秒泵浦-探测实验：通过一个由孤立阿秒紫外脉冲和强少周期红外脉冲组成的序列使 H(2)和 D(2)离解电离，并测量分子内电子电荷分布的局域化，该局域化依赖于泵浦和探测脉冲之间的延迟，具有阿秒时间分辨率。这种局域化是通过两种机制发生的，其中红外激光影响分子离子的光离化或离解。在第一种情况下，电荷局域化来自涉及自电离态和离去电子的激光改变波函数的量子力学干涉。在第二种情况下，电荷局域化是由于激光驱动的分子离子不同电子态之间的人口转移引起的。这些结果确立了阿秒泵浦-探测策略作为一种强大的工具，用于研究超出通常的 Born-Oppenheimer 近似的电子和核运动之间的复杂分子动力学。
