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。对于大约有20个原子的小银簇来说尤其如此。特别是支持的和自由的银原子团簇的光学性质是有趣的，目前对这种特殊的团簇系统的强烈兴趣是由于最近的两个观察：(i)氧化银粒子的照射导致光诱导形成表面支持的银原子团簇的稳定分布；（二）最近发现了一种强烈的可见荧光，源于银原子团簇中前所未有的长寿命激发态。这些特性使小原子银簇成为先进高密度光存储（通过(i)）和读出（通过（ii））设备的基础。然而，目前尚不清楚所观察到的荧光的详细电子来源和簇大小依赖关系，也没有关于银簇光还原形成机制的实验数据。为了深入了解原子银团簇的这些令人着迷的特性，我们打算将时间分辨光发射光谱技术应用于一个定义良好的团簇支撑系统，该系统由大量选择的银团簇软着陆在原子平面的镁质绝缘薄膜上组成。目标是确定支持银团簇的电子水平结构作为团簇大小的函数以及相应的激发态寿命，以便确定可能的辐射和非辐射跃迁。

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.