Excitons, quasiparticles of electrons and holes bound by Coulombic attraction, are created transiently by light and play an important role in optoelectronics, photovoltaics and photosynthesis. They are also predicted to form spontaneously in a small-gap semiconductor or a semimetal, leading to a Bose–Einstein condensate at low temperature, but there has not been any direct evidence of this effect so far. Here we detect the photoemission signal from spontaneously formed excitons in a debated excitonic insulator candidate, Ta₂NiSe₅. Our symmetry-selective angle-resolved photoemission spectroscopy reveals a characteristic excitonic feature above the transition temperature, which provides detailed properties of excitons, such as the anisotropic Bohr radius. The present result provides evidence for so-called preformed excitons and guarantees the excitonic insulator nature of Ta₂NiSe₅ at low temperature.

激子是由库仑引力束缚的电子和空穴的准粒子，由光瞬时产生，在光电子学、光伏和光合作用中发挥重要作用。还预测它们会在小间隙半导体或半金属中自发形成，从而在低温下形成玻色-爱因斯坦凝聚体，但迄今为止还没有任何直接证据表明这种效应。_{在这里，我们检测了来自有争议的激子绝缘体候选物 Ta 2} NiSe ₅中自发形成的激子的光电发射信号. 我们的对称选择角分辨光电子能谱揭示了高于转变温度的特征激子特征，这提供了激子的详细特性，例如各向异性玻尔半径。目前的结果为所谓的预形成激子提供了证据，并保证了 Ta ₂ NiSe ₅在低温下的激子绝缘体性质。