Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany.
Phys Rev Lett. 2012 Nov 16;109(20):207601. doi: 10.1103/PhysRevLett.109.207601. Epub 2012 Nov 13.
Femtosecond time-resolved two-photon photoemission spectroscopy is utilized to determine the electronically excited states dynamics at the α-sexithiophene (6T)/Au(111) interface and within the 6T film. We found that a photoinduced transition between the highest occupied molecular orbital and lowest unoccupied molecular orbital is essential in order to observe exciton population, which occurs within 100 fs. In thin 6T films, the exciton exhibits a lifetime of 650 fs. On a time scale of 400 fs, an energetic stabilization is observed leading to the formation of a polaron or electron trapping at defect states. The lifetime of this state is 6.3 ps. Coverage-dependent measurements show that apart from the excited state decay within the film, a substrate-mediated relaxation channel is operative. The present study demonstrates that two-photon photoemission spectroscopy is a powerful tool to investigate the whole life cycle from creation to decay of excitons in an organic semiconductor.
飞秒时间分辨双光子光电子能谱被用来确定α-六噻吩（6T）/金（111）界面和 6T 薄膜内的电子激发态动力学。我们发现，为了观察激子的形成，光诱导的最高占据分子轨道和最低未占据分子轨道之间的跃迁是必需的，这一过程发生在 100fs 内。在薄的 6T 薄膜中，激子的寿命为 650fs。在 400fs 的时间尺度上，观察到能量稳定化，导致极化子或电子在缺陷态的捕获。这个状态的寿命是 6.3ps。依赖于覆盖度的测量表明，除了在薄膜内的激发态衰减之外，还存在一个衬底介导的弛豫通道。本研究表明，双光子光电子能谱是一种强大的工具，可以研究有机半导体中激子从产生到衰减的整个生命周期。
