Measuring the Speed of Charge Migration in Molecules

Understanding how light interacts with molecules is a crucial step in various scientific disciplines. In particular, electron dynamics, which occur at the attosecond timescale, play a fundamental role in chemical reactions and biological functions related to light-matter interaction. One aspect of electron dynamics is charge migration (CM), which involves the movement of charge within a molecule. However, visualizing CM at the natural timescale of electrons is a challenging task due to the ultrafine spatial and ultrafast temporal resolution required.

Charge migration is a complex phenomenon because it is dependent on molecular orbitals and orientations. Experimental observations of CM dynamics have been challenging, and several questions regarding molecular CM still remain unanswered. One of the most fundamental questions is the speed at which charge migrates in molecules. While theoretical studies have contributed to our understanding of molecular CM, measuring the CM speed experimentally has proven to be extremely difficult.

Recently, a research team from Huazhong University of Science and Technology (HUST) collaborated with theoretical teams from Kansas State University and University of Connecticut to propose a method for measuring the CM speed in a carbon-chain molecule called butadiyne (C4H2). This method, known as high harmonic spectroscopy (HHS), is based on the three-step model of high-order harmonic generation (HHG).

The HHS method starts with ionization, where a strong field creates a hole wave packet in the ion. This wave packet evolves in the laser field and is probed by the returning electron wave packet during the recombination stage. The dynamics of the hole are recorded in the generated harmonic spectra. The researchers used a two-color HHS scheme in combination with an advanced machine learning reconstruction algorithm to reconstruct the CM in C4H2 at the most fundamental level for each fixed-in-space angle of the molecule. The method achieved an impressive temporal resolution of 50 attoseconds.

Quantifying CM Speed in C4H2

From the retrieved time-dependent hole densities, the researchers were able to identify the movement of the center of charge. This enabled them to quantify the CM speed, which was found to be several angstroms per femtosecond. Additionally, the dependence of CM speed on the alignment angles of the molecule with respect to the laser polarization was revealed. The researchers discovered that laser control can accelerate CM compared to the field-free case.

This groundbreaking work provides the first experimentally derived answer regarding the speed of CM in a molecule. It offers deep insights into CM dynamics in molecules and enhances our understanding of ultrafast dynamics. Professor Pengfei Lan, the corresponding author and a professor at the HUST School of Physics, remarks that this research not only contributes to our understanding of CM but also suggests a promising way to manipulate the rate of a chemical reaction through the control of CM speed. Professor Lan and his team plan to further explore this avenue in future studies.

The measurement of charge migration speed in molecules has been a long-standing challenge in ultrafast science. Through the use of high harmonic spectroscopy and machine learning reconstruction algorithms, the research team from HUST, in collaboration with theoretical teams from Kansas State University and University of Connecticut, successfully measured the CM speed in the carbon-chain molecule butadiyne. This achievement sheds light on the dynamics of CM in molecules and paves the way for potential applications in manipulating chemical reaction rates. The findings of this study mark a significant advancement in our understanding of ultrafast dynamics and open up new possibilities in the field of light-matter interactions.


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