在超快时间尺度上跟踪轨道图像
Tracing orbital images on ultrafast time scales
摘要：
前沿轨道，即，分子的最高占据轨道和最低未占据轨道通常决定分子性质，例如化学键和反应性。因此，人们对测量它们很感兴趣，尽管严格来说，它们不是量子力学的可观测量。然而，与光电子发射断层扫描技术最近已经推出了一个强大的技术，通过该技术，吸附在表面上的分子的轨道中的电子分布可以在动量空间中成像。这甚至被用于识别表面反应中的反应中间体。然而，到目前为止，还不可能及时跟踪轨道的动量-空间动力学，例如通过激发过程或化学反应。在这里，我们报告在这个方向上的关键一步：我们结合时间分辨的光电子发射采用高激光谐波和最近开发的动量显微镜，建立一个断层扫描，飞秒泵浦探测实验的未占用的分子轨道。具体来说，我们测量瞬态激发电子的全动量空间分布。因为在分子中，这种动量-空间分布与轨道形状密切相关，我们的实验提供了观察超快电子在时间和空间中运动的非凡可能性。这使我们能够将它们的激发态动力学与特定的实空间激发途径联系起来。
英文摘要：
Frontier orbitals, i.e., the highest occupied and lowest unoccupied orbitals of a molecule, generally determine molecular properties, such as chemical bonding and reactivities. Consequently, there has been a lot of interest in measuring them, despite the fact that, strictly speaking, they are not quantum-mechanical observables. Yet, with photoemission tomography a powerful technique has recently been introduced by which the electron distribution in orbitals of molecules adsorbed at surfaces can be imaged in momentum space. This has even been used for the identification of reaction intermediates in surface reactions. However, so far it has been impossible to follow an orbital's momentum-space dynamics in time, for example through an excitation process or a chemical reaction. Here, we report a key step in this direction: we combine time-resolved photoemission employing high laser harmonics and a recently developed momentum microscope to establish a tomographic, femtosecond pump-probe experiment of unoccupied molecular orbitals. Specifically, we measure the full momentum-space distribution of transiently excited electrons. Because in molecules this momentum-space distribution is closely linked to orbital shapes, our experiment offers the extraordinary possibility to observe ultrafast electron motion in time and space. This enables us to connect their excited states dynamics to specific real-space excitation pathways.
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