Collaborative Research: Probing Attosecond Charge Dynamics in Atoms and Molecules

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An ability to identify, tag and follow unambiguously quantum trajectories of electrons in molecular systems is central to addressing questions that are key to unlocking solutions to an array of contemporary scientific and technical challenges - understanding the chemistry of interstellar media, reducing our carbon footprint, enabling efficient, clean energy sources and controlling dynamics in biological molecules are just a few examples. Light-induced charge dynamics, for example, is ubiquitous in catalysis, photosynthesis, the photovoltaic effect, radiation damage in biomolecules and atmospheric chemistry. With clock speeds of order 1 femtosecond (10^-15 s), the energy flow and correlated dance between electrons and atomic nuclei require subfemtosecond temporal resolution (e.g., 100 attosecond = 0.1 femtosecond) and carefully-designed experimental techniques to track reliably. The University of Central Florida (UCF) - University of Maryland (UMD) collaboration has assembled the instrumentation and personnel to study such dynamics in important prototype molecules. Specifically, charge dynamics will be probed experimentally by transient changes in the absorption spectrum of so-called core-level states (those closest to the nucleus) of the carbon atom. These novel measurements may lead to new understanding of key physical concepts, clearer pictures of fundamental processes and novel ways to control electron dynamics. Employing a few-cycle infrared intense (IR) pump and an attosecond soft X-ray probe in the water window (240 to 330 eV in this study) for transient absorption of core-level carbon atoms, the University of Central Florida (UCF) - University of Maryland (UMD) collaboration is investigating ultrafast dynamics in two important hydrocarbons. One set of experiments is dedicated to tracking structural changes induced by the IR pulse in methane as it loses hydrogen atoms (deprotonization) with subfemtosecond temporal resolution. The second study focuses on IR-induced isomerization of acetylene into vinilydene also with subfemtosecond resolution. One novelty of these studies rests in a forty-fold improvement in temporal resolution over previous transient absorption measurements in the water window. The UCF-UMD collaboration is ideally suited to carry out this investigation because of the investigators' long history in building and operating state-of-the-art attosecond lasers (UCF) and probing and controlling atomic and molecular dynamics (UMD). To ensure the best interpretation of the experimental results, the team works closely with UCF theorists running contemporary numerical codes such as XCHEM and MESA to help analyze the data. The results will provide new insight into time-dependent structure changes and correlated electron motion induced by strong external perturbation, which potentially could reveal innovative ways to control electron dynamics.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.