Control and femtosecond time-resolved imaging of torsion in a chiral molecule

TL;DR

This study demonstrates how combined laser pulses can induce and monitor torsional motion in a chiral molecule in real-time, revealing detailed dynamics and correlations through femtosecond imaging and covariance analysis.

Contribution

It introduces a novel method of controlling and observing torsion in chiral molecules using femtosecond laser pulses combined with covariance analysis of Coulomb explosion images.

Findings

Torsion occurs with a 1.25 ps period and 3-degree amplitude.

Quantum dephasing leads to isotropic dihedral angle distribution over time.

Covariance analysis reveals strong correlations between ionic fragments.

Abstract

We study how the combination of long and short laser pulses, can be used to induce torsion in an axially chiral biphenyl derivative (3,5-difluoro-3',5'-dibromo-4'-cyanobiphenyl). A long, with respect to the molecular rotational periods, elliptically polarized laser pulse produces 3D alignment of the molecules, and a linearly polarized short pulse initiates torsion about the stereogenic axis. The torsional motion is monitored in real-time by measuring the dihedral angle using femtosecond time-resolved Coulomb explosion imaging. Within the first 4 picoseconds, torsion occurs with a period of 1.25 picoseconds and an amplitude of 3 degrees in excellent agreement with theoretical calculations. At larger times the quantum states of the molecules describing the torsional motion dephase and an almost isotropic distribution of the dihedral angle is measured. We demonstrate an original application of covariance analysis of two-dimensional ion images to reveal strong correlations between specific ejected ionic fragments from Coulomb explosion. This technique strengthens our interpretation of the experimental data.