Collaborative Research: Attosecond Charge Dynamics in Atoms and Molecules

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In the late 1800s, flash photography enabled the motion of macroscopic objects to be slowed to the point where the true order of events could be extracted. Nearly a century later, femtosecond pulses (1 femtosecond is one millionth of one billionth of a second) enabled the motions of atoms to be frozen during the formation and breakup of molecules, resulting in an improved microscopic picture of chemistry. The advent of attosecond pulses, a million times shorter, allows the motion of the tiniest building block of matter, the electron, to be stopped in its tracks. Professors Chang, of the University of Central Florida, and Hill, of the University of Maryland, together with their students, use attosecond laser pulses to study electronic charge dynamics on timescales commensurate with its indigenous motion in atomic and molecular systems. Charge migration, as it is known, is central to a variety of important processes such as photosynthesis, radiation damage in biomolecules and photovoltaics used in solar panels. This collaborative project is providing insight into the collective and interactive motion of electrons that are ubiquitous in atoms, molecules and solids. An improved understanding of charge migration will have broad application from controlling the flow of energy within a molecule to tailoring the performance of materials to specific needs.Photoinduced charge separation in molecules is the first step in many chemical processes and central to our understanding of electron correlation and the energy exchange between electronic and nuclear motion responsible for catalysis, photosynthesis, photovoltaics and radiation damage in biomolecules. Comprehensive numerical simulations of complex molecules predict that when an electron is 'suddenly' removed from one end of a chain molecule, such as a small peptide, the hole can move to the other end of the molecule in less than 10 fs, before the electron-nuclear coupling takes place. Intense, isolated attosecond pulses are required to study this naturally-occurring charge migration. Professors Chang and Hill, in a collaborative project between the University of Central Florida (UCF) and the University of Maryland (UMD), exploit the attosecond source at UCF to investigate charge migration in multi-electron atoms (He) and multi-atom molecules (SO2) via attosecond pump - attosecond probe experiments. The pump pulse initiates rapid excitation (in the absence of nuclear motion in the molecular case) while the probe pulse monitors the ensuing charge migration. Two probe techniques are used to 'watch' the charge migration: transient absorption and photoelectron angular distribution. In distinction with previous attosecond studies, where charge migration was investigated in the presence of a strong external infrared field, the UCF-UMD study is probing charge migration subsequent to excitation but in the absence of any external perturbation. As a consequence, this project is providing a clearer understanding of charge separation, energy flow and electron-nuclear charge coupling, which, as stated above, are relevant to a variety of processes associated with chemical reactions, dynamics in condensed matter systems and biological phenomena. A secondary goal of this project is the development of general experimental tools that can be transferred to more complicated systems, such as ABCU (1-azabicyclo [3.3.3] undecane), for which theoretical predictions exist and the excitation spectra fall near those of the model systems of this study.