Chinese team measures speed of molecular charge migration for the first time

The first step in discovering how light interacts with molecules is to follow the electron dynamics, which evolves on an attosecond time scale. The dynamics of this first step is known as charge migration (CM): CM can also be understood as the superposition of new eigenstates of cations in quantum mechanics when electrons are rapidly shed from a molecule creating an electronic wave packet (this is CM) that controls the flow of charge inside the molecule.

The CM plays a fundamental role in chemical reactions and biological functions associated with light-matter interactions. For many years, visualizing the CM on the natural time scale of the electron has been a formidable challenge in ultrafast science due to the need for ultrafine spatial (angstrom) and ultrafast temporal (attosecond) resolution.

In experiments, the sensitive dependence of CM on molecular orbitals and orientations makes CM dynamics complex and difficult to track. There are still a number of outstanding questions about molecular CM that remain unclear. One of the most fundamental questions is: how fast does charge migrate through a molecule? Despite extensive theoretical studies of molecular CM using time-dependent quantum chemistry packages over the past decade, practical measurements of CM velocities remain elusive due to the extremely challenging nature.

"Attosecond probing and control of charge migration in carbon-chain molecule."

A research team from Huazhong University of Science and Technology (HUST), in collaboration with theoretical teams from Kansas State University and the University of Connecticut, recently proposed a high-harmonic spectroscopy (HHS) method for measuring the CM velocity of the carbon-chain molecule butadiyne (C4H2), according to the journal Advanced Photonics.

The principle of the high-harmonic spectroscopy method is based on a three-step model of high-harmonic generation (HHG): ionization, acceleration, and recombination. Strong-field ionization first generates hole wave packets in the ions, which evolve in the laser field and are detected by the returning electron wave packets at the moment of recombination, and the hole dynamics are recorded in the resulting harmonic spectrum. The researchers used a two-color HHS scheme combined with advanced machine learning reconstruction algorithms to reconstruct the CM at the most fundamental level for each fixed spatial angle in the C4H2 molecule.It is worth noting that the temporal resolution of this method reaches 50as.

Probing CM in C4H2 using high-harmonic spectroscopy

Charge migration in the C4H2 molecule. (a) Time-dependent hole density reconstructed along the molecular backbone with the molecule perpendicular to the driving laser polarization direction. (b) Time-dependent charge center position reconstructed from the hole density in (a) (dashed line with circles). The red dashed line is a linear fit to the extracted CM velocity. (c)-(d) Same as (a)-(b) but with molecules aligned in parallel.

Based on the retrieved hole density over time, the shift of the charge center can be determined. From this it is possible to quantify the CM velocity, which is about a few angstroms per femtosecond. In addition, the relationship between the CM velocity and the alignment angle of the molecules with respect to the laser polarization was revealed. The results show that laser-controlled CM is faster than field-free CM. This work provides the first experimentally derived answer to the CM velocity in molecules.

In an interview, corresponding author Pengfei Lan, a professor at the School of Physics, Huazhong University of Science and Technology, said, "This work provides insight into the CM dynamics in molecules, which can enhance our understanding of these ultrafast dynamics." Lan Pengfei noted that controlling CM speeds through molecular arrangements is also a viable way to manipulate the rates of chemical reactions; his team hopes to explore this path in the near future.

Reference links:

[1]https://phys.org/news/2023-08-experimental-molecular-migration.html

[2]https://www.spiedigitallibrary.org/journals/advanced-photonics/volume-5/issue-05/056001/Attosecond-probing-and-control-of- charge-migration-in-carbon-chain/10.1117/1.AP.5.5.056001.full?SSO=1

[3] https://scitechdaily.com/charge-migration-measuring-the-speed-inside-molecules/

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