入射光子（或电子）的能量大于金属氧化物半导体带隙时，会将价带电子激发到导带产生的电子-空穴对，由于库仑相互作用电子-空穴对形成的类氢原子的准粒子，即激子。金属氧化物表面或界面的激子对光电/光伏器件，以及对金属氧化物表面的光催化过程有着重要的作用，引起了广发的关注和研究。我们在前期的研究中，观察到电致激发的金属氧化物表面激子对分子微观反应通道的奇特影响。本项研究将利用扫描隧道显微术/扫描隧道谱学（STM/STS）和超快时间分辨光电子能谱技术相结合的方法，研究金属氧化物表面吸附分子和多光子激发的氧化物表面激子的相互作用和寿命极其演化过程，探测光子诱导和电子诱导下激子的产生和扩散，激子对表面分子化学反应通道的影响，以及反应过程中能量传递通道和电荷转移的微观过程和可能的选键反应过程等。探测金属氧化物半导体中的激子效应，对深入研究光电/光伏器件、光催化反应有着重要的科学意义。

When the energy of incident photons (or electrons) is higher than the band gap of a metal-oxide semiconductor, the electrons in the valence band (VB) can be excited to the conduction band (CB) of the metal-oxide semiconductor, producing pairs of electron (in CB) and hole (in VB). The electron-hole pairs may form an electrically neutral quasiparticle as a bound state, i.e., exciton, because of the electrostatic Coulomb attraction. It is believed that the exciton states existed at the surfaces and/or interfaces of the metal oxides play a significant role in the photoelectric devices and photovoltaic cells, and also affect the processes of photocatalytic reactions. It has attracted extensive study interest in understanding the physical and chemical properties of excitons at the metal-oxide surfaces and interfaces. In our recent studies on the photocatalytic reactions of molecules on metal oxides, we have observed a surprising reaction route in microscopic studies. which could be attributed to the effect of the surface excitons. In this proposal, we plan to study the lifetime and the evolution of the excitons and their interactions with the adsorbed molecules on the oxide surfaces, by combined characterizations using scanning tunneling microscopy (STM) and the ultra-fast multiphoton photoemission (UFmPPE) technique. We intend to separately produce the excitons by photons and by injected electrons using the STM tip, respectively, and then compare the diffusion of excitons on surfaces, and the energy transfer channels of excitons to the adsorbed molecules, for in depth understanding of the affect of exictons on the photocatalytic reactions and the possible bond-selective processes of the molecules. Detecting and understanding the effects of exictons at metal-oxide surfaces and interfaces also have significant practical implications for the photoelectric and photovoltaic devices, and for the optimization of photocatalytic reactions.