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

Kansas State University, Manhattan KS

Investigators

Abstract

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.