Time-resolved photoemission from mass-selected metal clusters and their complexes with biomolecules at insulating surfaces

添加或移除一个原子会极大地改变小金属团簇的物理和化学性质。对于最多约有20个原子的小银团簇来说，情况尤其如此。特别是负载型和自由型银原子团簇的光学性质是很有趣的，最近的两个观测结果引起了人们对这一特殊团簇系统的强烈兴趣：(I)氧化银粒子的照射导致了表面支撑的银原子团簇的稳定分布的光诱导形成；(Ii)最近发现了强烈的可见荧光，来自于原子银团簇中前所未有的长寿命激发分子态。这些特性使小型原子银团簇成为先进的高密度光存储(通孔(I))和读出(通孔(II))设备的基础。然而，到目前为止，对于观察到的荧光的详细电子来源和团簇大小的依赖关系还不清楚，也没有关于银团簇的光还原形成机理的实验数据。为了深入了解银原子团簇的这些吸引人的性质，我们打算将时间分辨光电子能谱技术应用到一个定义明确的团簇支撑系统中，该系统由质量选择的银团簇软着陆在原子平坦的绝缘氧化镁薄膜上组成。我们的目标是确定作为团簇大小函数的银团簇的电子能级结构以及相应的激发态寿命，以便识别可能的辐射跃迁和非辐射跃迁。

Adding or removing one atom changes the physical and chemical properties of small metal clusters drastically. This is particularly true for small silver clusters with up to about 20 atoms. Especially the optical properties of supported and free atomic clusters of silver are intriguing and a strong current interest in this particular cluster system is due to two recent observations: (i) Irradiation of silver oxide particles leads to the photo-induced formation of stable distributions of surface-supported atomic silver clusters; (ii) a strong visible fluorescence was recently found, originating from unprecedented long-lived excited molecular states in atomic silver clusters. These properties uniquely qualify the small atomic silver clusters as a basis for advanced high-density optical storage (via (i)) and read-out (via (ii)) devices. However, neither the detailed electronic origin and cluster size dependence of the observed fluorescence is known up to now, nor any experimental data about the mechanism of the photoreductive formation of silver clusters. In order to get an insight into these fascinating properties of atomic silver clusters, we intend to apply the technique of time-resolved photoemission spectroscopy to a well-defined cluster-support system consisting of mass-selected silver clusters soft-landed onto an atomically flat insulating thin film of magnesia. The goal is to determine the electronic level structure of the supported silver clusters as a function of cluster size as well as the corresponding excited state lifetimes, in order to identify possible radiative and non-radiative transitions.