Frontier orbitals determine fundamental molecular properties such as chemical reactivities. Although electron distributions of occupied orbitals can be imaged in momentum space by photoemission tomography, it has so far been impossible to follow the momentum-space dynamics of a molecular orbital in time, for example, through an excitation or a chemical reaction. Here, we combined time-resolved photoemission using high laser harmonics and a momentum microscope to establish a tomographic, femtosecond pump-probe experiment of unoccupied molecular orbitals. We measured the full momentum-space distribution of transiently excited electrons, connecting their excited-state dynamics to real-space excitation pathways. Because in molecules this distribution is closely linked to orbital shapes, our experiment may, in the future, offer the possibility of observing ultrafast electron motion in time and space.

前沿轨道决定了基本的分子性质，例如化学反应性。虽然占据轨道的电子分布可以通过光电断层扫描在动量空间中成像，但到目前为止，还不可能及时跟踪分子轨道的动量空间动力学，例如通过激发或化学反应。在这里，我们结合使用高激光谐波的时间分辨光发射和动量显微镜来建立未占据分子轨道的断层扫描、飞秒泵浦探测实验。我们测量了瞬态激发电子的完整动量空间分布，将它们的激发态动力学与真实空间激发路径联系起来。因为在分子中，这种分布与轨道形状密切相关，我们的实验可能在未来，