Perspectives of Attosecond Spectroscopy for the Understanding of Fundamental Electron Correlations

The description of the electronic structure of molecules in terms of molecular orbitals is a highly successful concept in chemistry. However, it commonly fails if the electrons in a molecule are strongly correlated and cannot be treated as independent particles. Electron correlation is essential to understand inner‐valence X‐ray spectroscopies, it can drive ultrafast charge migration in molecules, and it is responsible for many exotic properties of strongly correlated materials. Time‐resolved spectroscopy with attosecond resolution is generally capable of following electronic motion in real time and can thus provide experimental access to electron‐correlation‐driven phenomena. High‐harmonic spectroscopy in particular uses the precisely timed laser‐driven recollision of electrons to interrogate the electronic structure and dynamics of the investigated system on a sub‐femtosecond timescale. In this Review, the capabilities of high‐harmonic spectroscopy to follow electronic motion in molecules are discussed. Both qualitative and quantitative approaches to unraveling the detailed dynamical responses of molecular systems following ionization are presented. A new theoretical formalism for the reconstruction of correlation‐driven charge migration is introduced. The importance of electron–ion entanglement and electronic coherence in the reconstruction of attosecond hole dynamics are discussed. These advances make high‐harmonic spectroscopy a promising technique to decode fundamental electron correlations and to provide experimental data on the complex manifestations of multi‐electron dynamics.