Understanding how light interacts with molecules is a crucial step in unraveling the mysteries of chemical reactions and biological functions associated with light-matter interaction. At the forefront of this exploration is the need to study electron dynamics, which occur at the incredibly fast attosecond timescale. The first step in studying electron dynamics is to investigate charge migration (CM), a process that plays a fundamental role in these interactions. However, visualizing CM at the natural timescale of electrons has proven to be a significant challenge due to the ultrafine spatial and ultrafast temporal resolution required.
For years, scientists have grappled with understanding the complex and elusive dynamics of charge migration in molecules, primarily because of its sensitive dependence on molecular orbitals and orientations. Despite significant theoretical advancements in the field, a real-time measurement of CM speed has remained elusive due to its extreme difficulty. However, a recent breakthrough by researchers from Huazhong University of Science and Technology (HUST), in collaboration with theoretical teams from Kansas State University and the University of Connecticut, has shed new light on this enigmatic process.
In their study, published in Advanced Photonics, the research team proposed a novel method called high harmonic spectroscopy (HHS) to measure CM speed in a specific carbon-chain molecule known as butadiyne (C4H2). HHS is based on the three-step model of high-order harmonic generation (HHG): ionization, acceleration, and recombination. By utilizing a two-color HHS scheme and an advanced machine learning reconstruction algorithm, the researchers were able to reconstruct the CM in C4H2 at the most fundamental level for every fixed-in-space angle of the molecule. This groundbreaking approach achieved an impressive temporal resolution of 50 attoseconds.
Through the analysis of time-dependent hole densities, the researchers were able to identify the movement of the center of charge in the molecule. From there, the CM speed was quantified, revealing a migration rate of several angstroms per femtosecond. Furthermore, the study also uncovered the dependence of CM speed on the alignment angles of the molecule with respect to the laser polarization. It was observed that CM under laser control exhibited a faster migration rate compared to the field-free scenario. This discovery marked the first time that a quantified, experimentally derived answer regarding CM speed in a molecule was achieved.
The breakthrough in measuring CM speed not only provides a deeper understanding of the dynamics at play in molecules but also opens up possibilities for manipulating the rate of chemical reactions. The ability to control CM speed through molecular alignment offers a promising avenue for influencing and optimizing chemical reactions. Professor Pengfei Lan, the corresponding author of the study and a professor in the HUST School of Physics, stated, “This work provides deep insight into CM dynamics in molecules and could strengthen our understanding of these ultrafast dynamics.” The research team intends to further explore the potential for manipulating the rate of chemical reactions through their newfound knowledge of CM speed.
The recent breakthrough achieved by the research team from HUST, Kansas State University, and the University of Connecticut provides a significant step forward in our understanding of charge migration dynamics in molecules. By developing the high harmonic spectroscopy method and successfully measuring CM speed in a specific carbon-chain molecule, they have unlocked crucial insights into ultrafast dynamics. This breakthrough not only enhances our fundamental understanding of chemical reactions but also opens up new possibilities for manipulating and optimizing these reactions. As researchers continue to delve into the intricacies of charge migration, we can expect further innovations and applications in the field of ultrafast science.
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