The dynamics of this very first action have actually been called charge migration (CM). Experimentally, the sensitive dependence of CM on molecular orbitals and orientations has made the CM characteristics complex and tough to trace. Molecular CM has been extensively studied in theory in the last decade by using time-dependent quantum chemistry bundles, a genuine measurement of the CM speed has stayed unattainable, due to the extreme obstacle.
Red dash-dotted line is a direct fitting to draw out the CM speed. The dependence of the CM speed on the alignment angles of the particle with respect to the laser polarization is also revealed.
Charge migration in a direct carbon-chain molecule, recorded by high harmonic spectroscopy (artistic making). Credit: Lixin He and Pengfei Lan
New speculative research for the very first time determines the speed of molecular charge migration.
Researchers developed a technique to determine charge migration speed in molecules, finding it can move several angstroms per femtosecond. The research study supplies insights into ultrafast molecular characteristics and possible chemical response control.
The dynamics of this very first step have been called charge migration (CM). For years, visualizing CM at the natural timescale of electrons has actually been a powerful difficulty in ultrafast science due to the ultrafine spatial (angstrom) and ultrafast temporal (attosecond) resolution required.
The Complexities and Challenges of Charge Migration
Experimentally, the sensitive dependence of CM on molecular orbitals and orientations has made the CM characteristics complex and tough to trace. There are still some open questions about molecular CM that remain unclear. One of the most essential questions: how quick does the charge migrate in particles? Although molecular CM has been thoroughly studied theoretically in the last years by utilizing time-dependent quantum chemistry packages, a real measurement of the CM speed has remained unattainable, due to the severe challenge.
Charge migration in a C4H2 molecule. (a) Time-dependent hole densities along the molecular backbone rebuilded for the perpendicular alignment of the particle with respect to the driving laser polarization instructions. (b) Time-dependent center of charge position (rushed line with circles) obtained from the hole densities in (a). Red dash-dotted line is a direct fitting to extract the CM speed. (c)-( d) Same as (a)-( b), however for the parallel alignment of the molecule. Credit: He et al., doi 10.1117/ 1. AP.5.5.056001.
Development Research on Measuring CM Speed.
As reported on August 24 in the journal Advanced Photonics, a research team from Huazhong University of Science and Technology (HUST), in cooperation with theoretical teams from Kansas State University and University of Connecticut, recently proposed a high harmonic spectroscopy (HHS) approach for measuring the CM speed in a carbon-chain particle, butadiyne (C4H2).
The principle of HHS is based upon the three-step model of high-order harmonic generation (HHG): acceleration, recombination, and ionization. Strong field ionization first develops a hole wave package in the ion, which evolves in the laser field and is probed by the returning electron wave package at the recombination moment, with the hole characteristics taped in the produced harmonic spectra.
The researchers utilized a two-color HHS scheme in mix with a sophisticated maker learning restoration algorithm to reconstruct the CM in C4H2 at the most fundamental level for each single fixed-in-space angle of the particle. The approach accomplished a temporal resolution of 50 as.
Discoveries and Future Implications.
From the recovered time-dependent hole densities, the movement of the center of charge is determined. From there, the CM speed is quantified, which has to do with a number of angstrom per femtosecond. The reliance of the CM speed on the positioning angles of the particle with respect to the laser polarization is likewise exposed. The CM under the laser control is demonstrated to be faster than the field-free one. This work for the very first time offers an experimentally derived response concerning the speed of CM in a molecule.
Corresponding author Pengfei Lan, a professor in the HUST School of Physics, remarks, “This work provides deep insight into CM characteristics in molecules and could strengthen our understanding of these ultrafast dynamics.”.
Lan notes that the control of CM speed by molecular alignment also suggests a promising method to manipulate the rate of a chain reaction– a course his group aims to check out in the future.
Referral: “Attosecond probing and control of charge migration in carbon-chain particle” by Lixin He, Yanqing He, Siqi Sun, Esteban Goetz, Anh-Thu Le, Xiaosong Zhu, Pengfei Lan, Peixiang Lu and Chii-Dong Lin, 24 August 2023, Advanced Photonics.DOI: 10.1117/ 1. AP.5.5.056001.
