Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan.
J Chem Phys. 2016 Dec 7;145(21):214703. doi: 10.1063/1.4968847.
Time resolved two-photon photoemission (TR-2PPE) spectroscopy has been performed for rubrene films on highly oriented pyrolytic graphite. When a second layer is formed on the first monolayer (ML), 2PPE intensity from the lowest unoccupied molecular orbital (LUMO)-derived level shows a clear resonance at a pump photon energy of 4.1 eV. In contrast, the resonance is very weak for sub-ML films. Substrate-molecule interaction blurs the intramolecular resonant transition for sub-ML films. The lifetime of electrons in the LUMO-derived level increases exponentially with increasing film thickness, for thickness up to 3 ML. The lifetime increase becomes more moderate for further increase in the film thickness. This change in the slope of the increase in lifetime suggests a transition in the relaxation mechanism, from electron tunneling to intramolecular relaxation medicated by the substrate. When ultraviolet photons of 4.45 eV are used to pump electrons to the LUMO-derived level, the decay profiles for films thicker than 1 ML deviate from a simple exponential decay. Such deviation is not significantly observed for sub-ML films. When visible photons of 2.97 eV are used for pumping, the decay profiles are well reproduced by a simple exponential decay, irrespective of the film thickness. The deviation from simple exponential decay is attributed to the relaxation of holes produced at deep occupied levels to the highest occupied molecular orbital-derived level.
时间分辨双光子光发射（TR-2PPE）光谱学已在高度取向的热解石墨上的芘膜上进行。当第二层形成在第一层单层（ML）上时，来自最低未占据分子轨道（LUMO）衍生能级的 2PPE 强度在 4.1eV 的泵浦光子能量下显示出明显的共振。相比之下，亚 ML 薄膜的共振非常弱。衬底-分子相互作用使亚 ML 薄膜的分子内共振跃迁变得模糊。LUMO 衍生能级中电子的寿命随薄膜厚度的增加呈指数增加，最大厚度可达 3 ML。对于进一步增加膜厚，寿命增加变得更加适中。寿命增加斜率的这种变化表明，从电子隧道到由衬底介导的分子内弛豫的弛豫机制发生了转变。当使用 4.45eV 的紫外线光子将电子泵浦到 LUMO 衍生能级时，厚度大于 1 ML 的薄膜的衰减曲线偏离简单指数衰减。对于亚 ML 薄膜，这种偏差不明显。当使用 2.97eV 的可见光光子进行泵浦时，衰减曲线无论薄膜厚度如何都可以通过简单的指数衰减很好地重现。偏离简单指数衰减归因于深占据能级产生的空穴松弛到最高占据分子轨道衍生能级。
