Achiral molecules have been prepared in states exhibiting chirality through control of their internal rotations. Manipulation of angular momentum within the molecule achieves this, then relying on external orientation as previous methods did. Either three microwave pulses or combinations of optical and THz pulses, all with mutually orthogonal polarization directions, induce the effect. A new technique generates chirality within molecules by controlling their internal rotations instead of external forces.

This method manipulates angular momentum inside the molecule itself to create asymmetry without directional fields or orientations. Chiral dynamics are detectable by observing how electrons are emitted from the molecule when using microwave and light pulses with specific polarization directions. Controlling molecular asymmetry has advanced via inducing chirality within achiral molecules through manipulation of their internal rotations. The approach moves beyond traditional techniques that rely on external forces, focusing instead on influencing angular momentum inside the molecule.

An intermediate state combines normally distinct left- and right-handed forms before resolving into one definite form; this illustrates chiral superposition states created during the experiment. Precisely timed pulses of microwave or light radiation achieve this effect, which is detectable via photoelectron circular dichroism similar to observing polarised light after interacting with molecules.

Molecular Chirality Induced via Controlled Rotational Angular Momentum

The technique central to these findings hinges on precisely shaped electromagnetic radiation, specifically sequences of microwave or combined optical and terahertz pulses. Carefully organised pulse structures manipulate a molecule’s rotational angular momentum, its spin within its framework, rather than applying external forces. This internal control circumvents limitations inherent in previous methods reliant upon aligning molecules before inducing chirality, offering greater command over molecular dynamics and enabling examination into time-odd chiroptical phenomena.

Either three distinct microwave bursts or combinations of light and THz waves possessing at least three mutually perpendicular polarisation directions created coherent rotations exhibiting handedness. The approach allows investigation into time-dependent effects even within randomly aligned samples, with chiral superpositions now creatable from initial states possessing only one non-zero dipole component, expanding possibilities for manipulating molecular rotation.

Microwave pulse control induces transient chiral states in achiral molecules

Achieving chiral superposition states in achiral molecules is now possible using just three microwave pulses, whereas previously inducing such asymmetry required external forces or pre-alignment which obscured microscopic origins of handedness. Manipulating a molecule’s internal rotations, its angular momentum, circumvents limitations inherent in earlier techniques reliant on laboratory frame orientation and establishes a new threshold for controlling chirality without directional bias. Precisely timed microwave bursts or combinations of optical and terahertz waves with mutually orthogonal polarisation directions created coherent rotational dynamics exhibiting detectable handedness via photoelectron circular dichroism.

Simulations confirmed sensitivity to purely rotational dynamics; measuring the emitted electrons’ polarisation reveals molecular chirality beyond structural properties. Symmetric top molecules, those with dipoles aligned along their symmetry axis, however, offer less control over generating chiral states as they require transitions perpendicular to this permanent dipole moment.

Internal molecular rotation induces temporary chirality despite limitations in aligned dipole systems

Controlling molecular handedness has long relied on inherent structural asymmetry or external forces acting upon molecules but a pathway to induce chirality through internal rotations alone now exists. Symmetric top molecules, where dipole moments align along their primary symmetry axis, prove more resistant to manipulation using these techniques because comparable control requires transitions perpendicular to the dominant dipole moment; this presents an immediate challenge for broadening applicability. This work establishes a method for creating temporary asymmetry within achiral molecules by manipulating their internal rotations and generating chiral superposition states without relying on external influences or pre-alignment. Detecting these fleeting arrangements involved measuring electron emission via photoelectron circular dichroism, revealing information about molecular rotation beyond simple structure; acknowledging that such control proves difficult with certain systems still represents strong progress.

The research demonstrated that chirality can be induced in achiral molecules through controlled rotational dynamics achieved using three microwave pulses or combinations of optical and terahertz radiation. This matters because it provides a new way to investigate the relationship between structural properties and dynamic behaviour when determining molecular handedness. Researchers used photoelectron circular dichroism to detect this temporary asymmetry created by internal rotations, offering insights into chiroptical phenomena without requiring external alignment. The authors suggest this method allows for distinguishing between chiral effects arising from static structures versus those generated during molecular motion.

👉 More information
🗞 Chiral rotational dynamics in the molecular frame: Breaking symmetry with angular momentum
✍️ Alexander Blech, Monika Leibscher and Christiane P. Koch
🧠 ArXiv: https://arxiv.org/abs/2608.17479

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