Charge Migration

K-State University Distinguished Professor Chii-Dong Lin, together with collaborators from Huazhong University and the University of Connecticut, recently proposed a high harmonic spectroscopy (HHS) method for measuring the charge migration (CM) speed in a carbon-chain molecule, butadiyne (C4H2). The principle of HHS is based on the three-step model of high-order harmonic generation (HHG): ionization, acceleration, and recombination. Strong field ionization first creates a hole wave packet in the ion, which evolves in the laser field and is probed by the returning electron wave packet at the recombination moment, with the hole dynamics 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 single fixed-in-space angle of the molecule. The method achieved a temporal resolution of 50 as. From analyzing the experimental results, they found that the charge migration speed ranges from 4.5 Å/fs to 2.9 Å/fs, for molecules oriented from parallel to perpendicular to the laser field direction. Here Å/fs ( Angstrom / femtosecond) is 100 km/s. For comparison, the speed of an Olympian sprinter is about 0.1 km/s.

This work was reported in the SPIE journal Advanced Photonics in August 2023 and in a SPIE press release, on which this description is based.