Reconstruction and control of a time-dependent two-electron wave packet

摘要

虽然两个或更多束缚电子的协同运动控制所有化学反应，但了解和探测这种电子动态仍有挑战性。单个电子的运动已以阿秒时间分辨率被观测到，但关于二电子运动的可比实验尚未实现。现在Christian Ott及同事发现，氦中两个关联电子的动态，可以从以前所未有的高光谱分辨率测出的阿秒瞬时吸收光谱、在一个强度可调的可见激光场存在的情况下重建。利用相同方法做进一步的实验，预计将能为验证理论提供基准数据，甚至也许还会使探测居于基本化学反应核心的亚稳电子跃迁态成为可能。%The concerted motion of two or more bound electrons governs atomic and molecular non-equilibrium processes including chemical reactions, and hence there is much interest in developing a detailed understanding of such electron dynamics in the quantum regime. However, there is no exact solution for the quantum three-body problem, and as a result even the minimal system of two active electrons and a nucleus is analytically intractable. This makes experimental measurements of the dynamics of two bound and correlated electrons, as found in the helium atom, an attractive prospect. However, although the motion of single active electrons and holes has been observed with attosecond time resolution, comparable experiments on two-electron motion have so far remained out of reach. Here we show that a correlated two-electron wave packet can be reconstructed from a 1.2-femtosecond quantum beat among low-lying doubly excited states in helium. The beat appears in attosecond transient-absorption spectra measured with unprecedentedly high spectral resolution and in the presence of an intensity-tunable visible laser field. We tune the coupling between the two low-lying quantum states by adjusting the visible laser intensity, and use the Fano resonance as a phase-sensitive quantum interferometer to achieve coherent control of the two correlated electrons. Given the excellent agreement with large-scale quantum-mechanical calculations for the helium atom, we anticipate that multidimensional spectroscopy experiments of the type we report here will provide benchmark data for testing fundamental few-body quantum dynamics theory in more complex systems. They might also provide a route to the site-specific measurement and control of metastable electronic transition states that are at the heart of fundamental chemical reactions.