Time-resolved low energy photoelectron diffraction for the study of surface structural dynamics with sub-100 fs temporal resolution

基于低能光电子的衍射，我们的目标是证明并应用一种新的实验概念来研究时间分辨率低于100fs的表面的超快结构动力学。除了结构信息，探测到的瞬时光电发射信号将同时携带有关所涉及的表面电子系统的激发和驰豫的信息，潜在地允许表面结构和载流子动力学的直接关联。在我们的研究中，我们将重点研究酞菁分子在贵金属表面的亚单层吸附。在静态ARPES实验中，这种吸附-表面系统的不同代表所报告的独特的低能光电子衍射图是检验所提出概念的能力和局限性的理想试验台。此外，我们期望结构动力学和载流子动力学的结合能够为控制这种金属-有机界面类的多方面性质的复杂的相互作用机制提供新的线索。在该项目中将实现和测试两种不同的实验配置：在使用70fsNUV激光脉冲(6 EV)来产生探测光电子时，我们将能够以低于100fs的最终时间分辨率探测结构和载流子动力学。然而，Ekin&lt；2 eV的本征极低的光电子动能将使我们的研究局限于小的和中等的泵浦通量，这是由于泵浦诱导的寄生电子发射产生的信号背景。在另一种配置中，我们将使用来自HHG源的18 eV真空紫外线脉冲，这样也可以在显著更高的泵浦通量下进行实验。在这一注量区，我们期望能够在吸附层中触发结构相变。

Based on the diffraction of low energy photoelectrons we aim for proving and applying a novel experimental concept for the study of ultrafast structural dynamics at surfaces with sub-100 fs temporal resolution. Next to structural information, the detected transient photoemission signal will at the same time carry information about excitation and relaxation of the involved surface electronic system potentially allowing for a direct correlation of structural and carrier dynamics at surfaces. Within our studies we will focus on sub-monolayer adsorption of phthalocyanine molecules on noble metal surfaces. The distinctive low energy photoelectron diffraction patterns that have been reported for different representatives of this adsorbate-surface system in static ARPES experiments represent an ideal test bed for the examination of the capabilities and limitations of the proposed concept. Furthermore, we expect that the combined interrogation of structural and carrier dynamics can shed new light into the complex interaction mechanisms governing the multifaceted properties of this metal-organic interface class. Two different experimental configurations will be realized and tested within the project: In using 70 fs NUV laser pulses (6 eV) for the generation of the probing photoelectrons we will be able to probe structural and carrier dynamics at an ultimate temporal resolution below 100 fs. The intrinsic very low kinetic energies of the photoelectrons of Ekin < 2 eV will, however, limit our studies to small and moderate pump fluences due to a signal background arising from pump-induced parasitic electron emission. In an alternative configuration we will use 18 eV vacuum ultraviolet pulses from a HHG source so that also experiments at significantly higher pump fluences will become possible. In this fluence regime we expect to be able to trigger also structural phase transitions in the adsorbate layer.