Mott insulators sometimes show dramatic changes in their electronic states after photoirradiation, as indicated by photoinduced Mott-insulator-to-metal transition. In the photoexcited states of Mott insulators, electron wave functions are more delocalized than in the ground state, and long-range Coulomb interactions play important roles in charge dynamics. However, their effects are difficult to discriminate experimentally. Here, we show that in a one-dimensional Mott insulator, bis(ethylenedithio)tetrathiafulvalene-difluorotetracyanoquinodimethane (ET-F₂TCNQ), long-range Coulomb interactions stabilize not only excitons, doublon-holon bound states, but also biexcitons. By measuring terahertz-electric-field-induced reflectivity changes, we demonstrate that odd- and even-parity excitons are split off from a doublon-holon continuum. Further, spectral changes of reflectivity induced by a resonant excitation of the odd-parity exciton reveals that an exciton-biexciton transition appears just below the exciton-transition peak. Theoretical simulations show that long-range Coulomb interactions over four sites are necessary to stabilize the biexciton. Such information is indispensable for understanding the non-equilibrium dynamics of photoexcited Mott insulators.

莫特绝缘子有时在光辐照后会显示出电子状态的巨大变化，如光诱导莫特-绝缘体到金属的转变所表明的那样。在Mott绝缘子的光激发态下，电子波函数比在基态下更离域，并且长距离库仑相互作用在电荷动力学中起重要作用。但是，它们的作用很难通过实验加以区分。在这里，我们表明在一维Mott绝缘体中，双（亚乙基二硫代）四硫富瓦烯-二氟四氰基喹二甲烷（ET-F ₂TCNQ），长距离库仑相互作用不仅可以稳定激子，杜布隆-霍隆结合态，而且还可以稳定双激子。通过测量太赫兹电场引起的反射率变化，我们证明了奇偶校验和激子是从杜宾-霍隆连续体中分离出来的。此外，由奇偶校验激子的共振激发引起的反射率的光谱变化表明，激子-比西克顿跃迁出现在激子跃迁峰的正下方。理论模拟表明，要稳定双激子，必须在四个位点进行长距离库仑相互作用。这样的信息对于理解光激发的Mott绝缘子的非平衡动力学是必不可少的。