Spectroscopy and Dynamics of Semiconductor, Metal, and Carbon Clusters using Photoelectron Imaging

本项目通过两种方法研究半导体、金属和碳团簇中的价带光谱和电子弛豫动力学：使用真空紫外（VUV）光源的阴离子光电子成像和飞秒时间分辨光电子成像。VUV实验将集中于绘制尺寸选择的磷化铟和硅半导体簇的价带结构。高能量的光分离，特别是在10.5电子伏特的情况下，将比以前更深入地探测到这些物质的价带。利用速度图成像（VMI）检测发射的光电子将提供光电子动能和角分布的同时测定，同时也区分直接光剥离和热离子发射。时间分辨光电子成像（TRPEI）实验将跟踪电子激发团簇中的弛豫路径。TRPEI将用于跟踪汞团簇中的内部转换和俄歇衰变动力学。它还将应用于磷化铟簇阴离子中的电子弛豫动力学，以及纯和金属掺杂的富勒化物。这个项目的成功需要批判性和独立思考的结合，以及广泛的实验室仪器的专业知识，包括真空、激光、电子和计算机编程。在这些广泛领域受过训练的学生在就业市场上具有很强的竞争力。团簇科学中提出的研究计划侧重于物质的性质，特别是半导体和金属，如何在分子和体尺寸之间演变。因此，它为理解纳米科学和纳米技术的基础提供了一个基本框架。纳米技术在基础研究和整个社会中的重要性与日俱增，这激发了人们对集群科学基本问题的极大兴趣。纳米科学关注的是大块材料的性质在纳米尺度下的不同，而簇科学传统上关注的是原子和分子的性质在聚集过程中是如何演变的。这种方法，其中一个研究变化的几何和光谱团随着尺寸的增加，已经非常富有成效。然而，近年来，人们对一个平行问题很感兴趣，即一个团簇必须多大才能观察到通常与块状材料相关的类似现象，如半导体中的能带结构、金属-绝缘体跃迁、液体中的电子溶剂化和热离子发射。拟议的研究将解决其中的许多问题。它还将通过为本科生、研究生和博士后水平的年轻研究人员提供最先进的研究培训，为美国的科学基础设施做出重大贡献。这个NSF项目是由化学部门和材料研究部门共同资助的。

This project addresses valence band spectroscopy and electronic relaxation dynamics in semiconductor, metal, and carbon clusters investigated by two methods: anion photoelectron imaging using vacuum ultraviolet (VUV) light sources, and femtosecond time-resolved photoelectron imaging. The VUV experiments will focus on mapping out the valence band structure of size-selected indium phosphode and silicon semiconductor clusters. Photodetachment at high energies, particularly at 10.5 electron-volts, will probe more deeply into the valence bands of these species than has been previously possible. Detection of the ejected photoelectrons using velocity-map imaging (VMI) will provide simultaneous determination of photoelectron kinetic energy and angular distributions, while also discriminating between direct photodetachment and thermionic emission. The time-resolved photoelectron imaging (TRPEI) experiments will track relaxation pathways in electronically excited clusters. TRPEI will be used to follow internal conversion and Auger decay dynamics in mercury clusters. It will also be applied to electronic relaxation dynamics in indium phosphide cluster anions, and in pure and metal-doped fullerides. Success on this project requires a combination of critical and independent thinking along with expertise in a wide range of laboratory instrumentation, including vacuum, lasers, electronics, and computer programming. Students trained in these broad areas are highly competitive in the job market.%%%The proposed research program in cluster science focuses on how the properties of matter, particularly semiconductors and metals, evolve between the molecular and bulk size regimes. As such, it provides a fundamental framework for understanding the foundations of nanoscience and nanotechnology. The increasing emphasis on nanotechnology in basic research and society as a whole has stimulated much interest in fundamental questions of cluster science. While nanoscience focuses on how the properties of bulk materials differ in the nanoscale regime, cluster science has traditionally focused on how the properties of atoms and molecules evolve upon aggregation. This approach, in which one investigates changes in the geometry and spectroscopy of clusters with increasing size, has been extremely productive. However, in recent years, there has been much interest in a parallel question, namely how large does a cluster have to be in order to observe the analog of phenomena normally associated with bulk materials, such as band structure in semiconductors, metal-insulator transitions, electron solvation in liquids, and thermionic emission. The proposed research will address many of these issues. It will also contribute significantly to the scientific infrastructure of the U.S. by providing state-of-the-art research training to young researchers at the undergraduate, graduate, and post-doctoral levels. This NSF project is being co-funded by the Chemistry Division and the Division of Materials Research.