[Paper Review] Manipulating the torsion of molecules by strong laser pulses

C. B. Madsen, Lars Bojer Madsen+7 more|K-State Research Exchange (Kansas State University)|Sep 17, 2008

Laser-Matter Interactions and Applications3 citations

TL;DR

This study demonstrates laser-induced control of torsional motion in isolated 3,5-difluoro-3',5'-dibromobiphenyl (DFDBrBPh) molecules using a sequence of nanosecond and femtosecond laser pulses. A 9 ns 1064 nm pulse aligns the C–C bond axis, followed by a 700 fs 800 nm perpendicular kick pulse that induces coherent torsional oscillation, monitored via femtosecond Coulomb explosion imaging, with theory confirming a 1.2 ps oscillation period and 2.45° amplitude in the dihedral angle.

ABSTRACT

A proof-of-principle experiment is reported, where torsional motion of a molecule, consisting of a pair of phenyl rings, is induced by strong laser pulses. A nanosecond laser pulse spatially aligns the carbon-carbon bond axis, connecting the two phenyl rings, allowing a perpendicularly polarized, intense femtosecond pulse to initiate torsional motion accompanied by an overall rotation about the fixed axis. The induced motion is monitored by femtosecond time-resolved Coulomb explosion imaging. Our theoretical analysis accounts for and generalizes the experimental findings.

Motivation & Objective

• To experimentally demonstrate control of torsional motion in a chiral biphenyl derivative using strong-field laser pulses.
• To develop a method for time-resolved observation of ultrafast torsional dynamics in isolated molecules.
• To enable future applications in molecular electronics and enantioselective chemistry via laser-induced torsional switching.
• To provide a theoretical framework for understanding the induced torsional potential modification and wave packet dynamics.
• To explore the feasibility of laser-driven de-racemization by breaking inversion symmetry through molecular orientation.

Proposed method

• A 9 ns, 1064 nm linearly polarized laser pulse aligns the C–C bond axis of DFDBrBPh molecules in the lab frame via non-resonant dipole forces.
• A subsequent 700 fs, 800 nm, 5×10¹² W/cm² laser pulse, polarized perpendicular to the aligned axis, acts as a 'kick' to induce torsional motion in the dihedral angle between phenyl rings.
• A 25 fs, 800 nm, 2×10¹⁴ W/cm² probe pulse triggers Coulomb explosion, ionizing the molecule and allowing two-dimensional ion imaging of F⁺ and Br⁺ fragments to reconstruct ring orientation.
• Angular distributions of ion fragments are analyzed to extract time-dependent dihedral angles, with peak splitting used to quantify torsional motion.
• Theoretical simulations model the time evolution of the dihedral angle φd using a double-well torsional potential, accounting for wave packet dynamics and pulse-induced potential modification.
• Extended simulations explore enantioselective switching by applying a 1.0 ps, 1.2×10¹³ W/cm² kick pulse to pre-oriented molecules, with 99% conversion from Rₐ to Sₐ enantiomer.

Experimental results

Research questions

• RQ1 Can strong-field laser pulses induce coherent torsional motion in a chiral biphenyl molecule without resonant excitation?
• RQ2 To what extent can the torsional potential of a molecule be transiently modified by non-resonant laser fields to drive controlled molecular motion?
• RQ3 Can femtosecond Coulomb explosion imaging resolve time-dependent torsional dynamics with sub-picosecond precision?
• RQ4 Is it possible to achieve enantioselective switching of chiral molecules using tailored laser pulses and molecular orientation?
• RQ5 How does the dihedral angle evolve after a laser kick, and can theoretical models accurately reproduce the observed oscillations?

Key findings

• The experiment observes a 1.2 ps oscillation period in the dihedral angle φd, matching the theoretical prediction of ~1.2 ps and consistent with the experimental value of ~1 ps.
• The amplitude of torsional oscillation is measured at ~2.45° in theory, with a smaller modulation of ~0.6° expected in experiment due to lack of initial 3D alignment.
• The F⁺ ion angular distribution shows a clear four-peak structure at tₚ = 1.47 ps, indicating coherent torsional motion, with splitting corresponding to the average angle between ion recoil and kick pulse polarization.
• At tₚ = 2.47 ps, the angular distribution broadens, indicating dephasing and loss of coherence, consistent with theoretical predictions of wave packet spreading.
• Theoretical simulations show that 99% of Rₐ enantiomers convert to Sₐ upon laser kick, while only 13% of Sₐ enantiomers convert to Rₐ, indicating strong enantioselectivity.
• The method enables time-resolved observation of de-racemization, with potential for ultrafast molecular switching in molecular junctions on a picosecond timescale.