Imaging Electronic Dynamics in Matter with Atomic Spatio-Temporal Resolution.

光电子发射是自然界中最基本的过程之一。当足够短波长和足够强度的电磁辐射击中目标，通过入射辐射与目标中的电子进行耦合，并导致光子能量转移到目标的内部激发和电子发射时，就会发生这种情况。当这些光电子离开目标时，它们携带了关于光电子动力学以及目标材料的电子性质的信息。一个多世纪以来，对其能量和动量分布的测量和分析一直是确定物质电子结构的主要方法，对激光和探测技术以及精确的量子力学理论方法的发展起到了重要的推动作用。几十年来，基于光源的能域光电子能谱和不允许电子动力学的时间分辨率的检测方案一直被常规使用，并且仍然是成像电子结构的首选工具。能域谱可以反映样品在光电子发射过程中的时间平均内部电子动力学，但不能分辨光电子释放过程中的超快随时间变化的电子动力学。这项拟议的理论工作的动机是，在过去20年里，超快激光技术取得了非凡的进步，使超短光脉冲的产生及其精确控制和同步成为可能。这些脉冲允许研究原子、分子和凝聚物质系统中的电子动力学，其时间分辨率为物质中电子运动的自然时间尺度(约十亿分之一秒)。就像拍摄快速移动的物体(如赛车或飞行中的子弹)的电影需要许多帧的频闪组装一样，每一帧都构成了物体的瞬时图像，时域光谱学即将允许合成电子电影，能够显示例如化学键的形成和断裂。这些研究可能会对新兴技术产生决定性的影响，如光波计算、纳米催化和人工光合作用，有助于开发新的计算机和催化设备，以确保我们的能源供应和保护我们的环境。几秒(1As=10-18秒)时间分辨光谱学导致了令人印象深刻的对孤立(气体)原子电离过程的时间分辨研究，并有望显著提高我们对分子、层状半导体结构和纳米粒子的电子性质的理解。然而，时间分辨光电子能谱的详细物理解释面临着重大的概念挑战，需要进行全面的理论研究，即使是对简单的原子系统也是如此。对于复杂的系统，如大分子、纳米粒子和固体表面，必须克服在描述瞬时光激电子动力学方面的额外严重的技术困难。该项目解决了这些挑战，重点是对吸附金属表面和纳米颗粒的时间和空间分辨光电子发射进行建模。它通过发展和应用互补的量子力学方法来进行，包括依赖时间的薛定谔方程的精确数值解和物理上更透明的分析方法。它将评估时间和光电子发射角度分辨光谱能够揭示以下信息的保真度：(A)固体中的电子力和动力学，以及(B)响应入射红外或可见光脉冲的非均匀纳米等离子体电场增强。拟议的复杂系统时空分辨研究将促进对(I)基本过程，如单电子和集体电子激发，以及(Ii)层状半导体、吸附表面、生物分子和纳米粒子中电子和场的动力学的理解。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Photoelectron emission is one of the most fundamental processes in nature. It occurs when electromagnetic radiation of sufficiently short wavelength and adequate intensity strikes a target, proceeds through the coupling of the incident radiation with electrons in the target, and results in the transfer of photonic energy to internal excitations of the target and the emission of electrons. When these photoelectrons leave the target, they carry information about the photoemission dynamics as well as the electronic properties of the target material. For more than a century, the measurement and analysis of their energy and momentum distribution has been a primary method for determining the electronic structure of matter, importantly promoting the development of laser and detection technologies as well as accurate quantum-mechanical theoretical methods. Energy-domain photoelectron spectroscopy based on light sources and detection schemes that do not allow time-resolution of the electronic dynamics have been routinely employed for many decades and remain a preferred tool for imaging electronic structure. Energy-domain spectra can image the sample's time-averaged internal electronic dynamics during the photoemission process, but do not resolve the ultrafast time-dependent electronic dynamics during the photoelectron-release process. The proposed theoretical work is motivated by extraordinary progress in ultrafast laser technology over the past two decades that enabled the generation of ultrashort light pulses and their accurate control and synchronization. These pulses allow for investigations of the electronic dynamics in atoms, molecules, and condensed matter systems with temporal resolution at the natural timescale of the electronic motion in matter (of the order of a billionth of a billionth of a second). In same way as making a movie of a fast-moving object, such as a race car or a bullet in flight requires the stroboscopic assembly of many frames, each constituting a momentary image of the object, time-domain spectroscopy is about to allow the composition of electronic movies, capable of displaying, for example, the formation and breaking of chemical bonds. These investigations may have a decisive impact on emerging technologies, such as light-wave computing, nano-catalysis, and artificial photosynthesis, contributing to the development of novel computers and catalytic devices for securing our energy supply and preserving our environment.Attosecond (1 as = 10-18 seconds) time-resolved spectroscopy has led to impressive time-resolved studies of ionization processes on isolated (gaseous) atoms and is anticipated to significantly advance our understanding of electronic properties of molecules, layered-semiconductor structures, and nanoparticles. However, the detailed physical interpretation of time-resolved photoemission spectra faces significant conceptual challenges and necessitates comprehensive theoretical investigations, even for simple atomic systems. For complex systems, such as large molecules, nanoparticles, and solid surfaces, additional severe technical difficulties in describing the transiently photoexcited electronic dynamics must be overcome. This project addresses these challenges and focuses on the modeling of time- and spatially resolved photoemission from adsorbate-covered metal surfaces, and nanoparticles. It proceeds by developing and applying complementary quantum-mechanical methods, including exact numerical solutions of the time-dependent Schroedinger equation and physically more transparent analytical methods. It will assess the fidelity with which time- and photoelectron-emission-angle-resolved spectra can reveal information on (a) electronic forces and dynamics in solids and (b) non-homogenous nano-plasmonic electric-field enhancements in response to incident infrared or visible light pulses. The proposed spatio-temporally-resolved studies of complex systems will promote the understanding of (i) elementary processes, such as single--electron and collective electronic excitations, and (ii) the dynamics of electrons and fields in layered semiconductors, adsorbate-covered surfaces, biomolecules, and nanoparticles.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.