Ino Daisuke, Watanabe Kazuya, Takagi Noriaki, Matsumoto Yoshiyasu
Department of Photoscience, School of Advanced Sciences, The Graduate University for Advanced Studies (Sokendai), Hayama, Kanagawa 240-0193, Japan.
J Phys Chem B. 2005 Sep 29;109(38):18018-24. doi: 10.1021/jp052078d.
The femtosecond time evolutions of excited states in zinc phthalocyanine (ZnPC) films and at the interface with TiO2(110) have been studied by using time-resolved two-photon photoelectron spectroscopy (TR-2PPE). The excited states are prepared in the first singlet excited state (S1) with excess vibrational energy. Two different films are examined: ultrathin (monolayer) and thick films of approximately 30 A in thickness. The decay behavior depends on the thickness of the film. In the case of the thick film, TR-2PPE spectra are dominated by the signals from ZnPC in the film. The excited states decay with tau = 118 fs mainly by intramolecular vibrational relaxation. After the excited states cascaded down to near the bottom of the S1 manifold, they decay slowly (tau = 56 ps) although the states are located at above the conduction band minimum of the bulk TiO2. The exciton migration in the thick film is the rate-determining step for the electron transfer from the film to the bulk TiO2. In the case of the ultrathin film, the contribution of electron transfer is more evident. The excited states decay faster than those in the thick film, because the electron transfer competes with the intramolecular relaxation processes. The electronic coupling with empty bands in the conduction band of TiO2 plays an important role in the electron transfer. The lower limit of the electron-transfer rate was estimated to be 1/296 fs(-1). After the excited states relax to the states whose energy is below the conduction band minimum of TiO2, they decay much more slowly because the electron-transfer channel is not available for these states.
利用时间分辨双光子光电子能谱（TR-2PPE）研究了酞菁锌（ZnPC）薄膜及其与TiO₂(110)界面处激发态的飞秒时间演化。激发态是在具有过量振动能量的第一单线态激发态（S1）中制备的。研究了两种不同的薄膜：超薄（单层）薄膜和厚度约为30 Å的厚膜。衰减行为取决于薄膜的厚度。对于厚膜，TR-2PPE光谱主要由薄膜中ZnPC的信号主导。激发态以τ = 118 fs的时间常数主要通过分子内振动弛豫衰减。在激发态级联下降到S1多重态底部附近后，尽管这些态位于块状TiO₂导带最小值之上，但它们衰减缓慢（τ = 56 ps）。厚膜中的激子迁移是电子从薄膜转移到块状TiO₂的速率决定步骤。对于超薄薄膜，电子转移的贡献更为明显。激发态的衰减比厚膜中的更快，因为电子转移与分子内弛豫过程相互竞争。与TiO₂导带中空带的电子耦合在电子转移中起重要作用。电子转移速率的下限估计为1/296 fs⁻¹。在激发态弛豫到能量低于TiO₂导带最小值的态之后，它们衰减得更慢，因为这些态没有电子转移通道。
