Attosecond Electron Dynamics

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

In this project funded by the Chemical Structure Dynamics and Mechanism-A (CSDM-A) program of the Chemistry Division, Professor Stephen Leone of the University of California Berkeley is using innovative laser techniques to investigate the behavior of molecules at the shortest possible time. The experiments detect fast molecular changes on timescales down to hundreds of attoseconds (1 attosecond is one billionth of a billionth of a second). Such measurements have only become possible in the last 15 years due to new methods for producing attosecond light pulses with lasers. Because electrons move so rapidly, their motion (electron dynamics) becomes central to the molecular processes under investigation. This research measures fast electron motion, such as when charges redistribute or periodically change in molecules, or electrons hop as the structure of the molecule transforms. To accomplish this, Professor Leone produces two laser pulses, one to excite the molecules and one delayed in time to measure the molecular response, analogous to a starting pistol and a stopwatch in a race. These experiments use numerous optical technologies to obtain the required short pulses and accurate time delays. The benefits to society are anticipated from the development of measurement capabilities that push the boundaries of time using tools that require precision stability. These techniques are important as the dimensions of devices decrease and performance speeds of storage media and computational tools increase. The students engaged in this project are learning an array of techniques and principles relevant for high technology professions, including laser technology, electronics, and computing. Attosecond time-resolved measurements represent a new way to probe chemical dynamics on timescales short enough to separate electron dynamics from nuclear motion. Electron dynamics such as electron correlation and electronic superposition states play a central role in chemical processes on these short timescales, as does the breakdown of the separability of timescales between electrons and nuclei (Born Oppenheimer). To study these phenomena, an experimental laboratory based on the production of isolated attosecond pulses in the extreme ultraviolet (XUV) spectral range is employed. The chemical systems involve measurements of few-femtosecond and subfemtosecond (attosecond) time dynamics of electronic superpositions, dissociation processes, and passage through curve crossings or conical intersections of electronically excited molecules. The students involved in this attosecond measurement-based project are gaining experience in a variety of areas that are emerging in high tech industry. These include, for example, carrier-envelope-phase stabilized lasers, interferometric control of light, core level spectroscopy principles, x-ray optical programming, electron spectrometers, and rate equation approaches to predict ionization, orbital occupancy, and alignment.