An excitonic insulator phase is expected to arise from the spontaneous formation of electron–hole pairs (excitons) in semiconductors where the exciton binding energy exceeds the size of the electronic band gap. At low temperature, these ground state excitons stabilize a new phase by condensing at lower energy than the electrons at the valence band top, thereby widening the electronic band gap. The envisioned opportunity to explore many-boson phenomena in an excitonic insulator system is triggering a very active debate on how ground state excitons can be experimentally evidenced. Here, we employ a nonequilibrium approach to spectrally disentangle the photoinduced dynamics of an exciton condensate from the entwined signature of the valence band electrons. By means of time- and angle-resolved photoemission spectroscopy of the occupied and unoccupied electronic states, we follow the complementary dynamics of conduction and valence band electrons in the photoexcited low-temperature phase of Ta₂NiSe₅, the hitherto most promising single-crystal candidate to undergo a semiconductor-to-excitonic-insulator phase transition. The photoexcited conduction electrons are found to relax within less than 1 ps. Their relaxation time is inversely proportional to their excess energy, a dependence that we attribute to the reduced screening of Coulomb interaction and the low dimensionality of Ta₂NiSe₅. Long after ($>$ 10 ps) the conduction band has emptied, the photoemission intensity below the Fermi energy has not fully recovered the equilibrium value. Notably, this seeming carrier imbalance cannot be rationalized simply by the relaxation of photoexcited electrons and holes across the semiconductor band gap. Rather, a rate equation model involving different photoemission crosssections of the valence electrons and the condensed excitons is able to reproduce the delayed recovery of the photoemission intensity below the Fermi energy. The model shows that electron quantum tunnelling between the exciton condensate and the valence band top is enabled by an extremely small activation energy of $4\times {10}^{-6}$ eV and explains the retarded recovery of the exciton condensate. Our findings not only determine the energy gain of ground state exciton formation with exceptional energy resolution, but also demonstrate the use of time-resolved photoemission to unveil the re-formation dynamics of an exciton condensate with femtosecond time resolution.

激子绝缘体相预计是由半导体中电子-空穴对（激子）的自发形成引起的，其中激子结合能超过电子带隙的大小。在低温下，这些基态激子通过以比价带顶电子更低的能量凝聚来稳定新相，从而扩大电子带隙。在激子绝缘体系统中探索多玻色子现象的设想机会正在引发一场关于如何通过实验证明基态激子的非常活跃的辩论。在这里，我们采用非平衡方法从价带电子的缠绕特征中光谱解开激子凝聚体的光致动力学。₂ NiSe ₅，迄今为止最有前途的单晶候选物，可以进行半导体到激子绝缘体的相变。发现光激发传导电子在小于 1 ps 内弛豫。它们的弛豫时间与其过剩能量成反比，我们将这种依赖性归因于库仑相互作用的屏蔽减少和 Ta ₂ NiSe ₅的低维数。很久以后 （$>$10 ps)导带已经排空，费米能级以下的光发射强度还没有完全恢复平衡值。值得注意的是，这种看似载流子的不平衡不能简单地通过半导体带隙中光生电子和空穴的弛豫来解释。相反，涉及价电子和凝聚激子的不同光发射横截面的速率方程模型能够再现低于费米能量的光发射强度的延迟恢复。该模型表明，激子凝聚体和价带顶之间的电子量子隧穿是由极小的活化能实现的$\mathrm{4个}\times {10}^{-\mathrm{6个}}$eV 并解释了激子凝聚物的延迟恢复。我们的研究结果不仅确定了具有出色能量分辨率的基态激子形成的能量增益，而且还证明了使用时间分辨光电发射来揭示具有飞秒时间分辨率的激子凝聚体的重新形成动力学。