The interaction between light and molecules is a complex phenomenon that plays a crucial role in chemical reactions and biological functions. To truly understand this intricate process, scientists must delve into the dynamics of electron behavior, which occur on an incredibly short timescale known as the attosecond. One of the most important steps in this process is charge migration (CM), which has posed significant challenges in terms of visualization and measurement due to its ultrafine spatial and ultrafast temporal resolution requirements. However, a recent groundbreaking study conducted 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 introduced a high harmonic spectroscopy (HHS) method that enables the measurement of CM speed in molecules. This article explores the details of this innovative approach and its implications for understanding ultrfast dynamics in molecules.
The HHS method developed by the research team is based on the three-step model of high-order harmonic generation (HHG). The process begins with strong field ionization, which creates a hole wave packet in the ion. This wave packet then evolves in the laser field and is probed by the returning electron wave packet during recombination. The dynamics of the hole are recorded in the generated harmonic spectra. By utilizing a two-color HHS scheme and an advanced machine learning reconstruction algorithm, the researchers were able to reconstruct the CM in a carbon-chain molecule called butadiyne (C4H2) at the most fundamental level for each fixed-in-space angle of the molecule. This innovative approach provided a temporal resolution of 50 attoseconds, allowing for unprecedented insights into CM dynamics.
Quantifying CM Speed
Through the analysis of time-dependent hole densities, the researchers were able to identify the movement of the center of charge and quantify the CM speed in butadiyne. The results revealed that the CM speed is approximately several angstroms per femtosecond. Furthermore, the study uncovered the dependence of CM speed on the alignment angles of the molecule with respect to the laser polarization. It was demonstrated that CM under laser control is faster than in a field-free environment. This groundbreaking research represents the first experimental measurement of CM speed in a molecule, providing valuable insights into the fundamental dynamics of charge migration.
The ability to measure and understand CM dynamics in molecules opens up a world of possibilities for advancements in ultrfast science. Professor Pengfei Lan, the corresponding author of the study and a professor in the HUST School of Physics, believes that this work provides deep insights into CM dynamics and strengthens our understanding of ultrafast processes. Additionally, the control of CM speed through molecular alignment offers a promising avenue for manipulating the rate of chemical reactions. This exciting prospect is a path that Professor Lan and his team aim to explore in the near future.
The development of the high harmonic spectroscopy method has revolutionized the study of charge migration in molecules. By overcoming the challenges presented by ultrafine spatial and ultrafast temporal resolution requirements, researchers have successfully quantified the CM speed in a molecule for the first time. This achievement provides invaluable insights into the fundamental dynamics of CM and contributes to a deeper understanding of ultrfast processes in the field of science. Furthermore, the control of CM speed through molecular alignment offers the potential to manipulate chemical reactions, opening up new possibilities for advancements in various fields. The future of ultrfast science is undoubtedly bright, thanks to the groundbreaking work of the research team from HUST, Kansas State University, and the University of Connecticut.
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