Collisions between dipolar molecules ‘switch off’ at low energies

Researchers in the Netherlands have demonstrated a method to deactivate the dipole moment of typically polar molecules during low-energy collisions. This groundbreaking discovery, achieved by overcoming significant experimental challenges, holds implications for ultracold chemistry and may contribute to a more profound understanding of polarity, benefiting both quantum simulation and computation.

Molecular interactions are often intricate and chaotic, making their study at the atomic and molecular level a daunting task. To address this, scientists typically cool reactants to just above absolute zero, which limits their movement and vibrations. However, interactions between ultracold molecules with an electric dipole moment remain poorly understood, particularly when these molecules possess a permanent charge distribution.

In 2023, researchers Bas van de Meerakker and his theorist colleagues at Radboud University Nijmegen, Netherlands, studied the collision cross-section between ammonia molecules (NH3) at low energies. Their simulations revealed that this cross-section decreases sharply as energy decreases, defying the classical view that it should continuously increase.

This discrepancy can be attributed to the quantum mechanical depiction of a specific quantum state in a molecule as possessing a definite parity, or symmetry. When polar molecules collide at high speeds, their opposite parity states intermingle through electrostatic field interactions, effectively "turning on" each other's dipole moments, which enhances collision cross-sections.

At lower speeds, however, the molecules cannot approach closely enough for this mutual mixing to occur. Consequently, their dipole moments effectively "switch off," leading to significantly reduced collision rates. To experimentally validate this phenomenon, the researchers had to devise a method to overlap two molecular beams, which generally requires using electric fields to merge the beams.

The risk of such an approach is that the beams may never overlap due to the electric field's influence. The Radboud team overcame this obstacle by integrating a 2.6 m Stark decelerator, a curved hexapole guide, and a merged quadrupole/hexapole trap to create a device that can precisely align the beams at the right position with minimal angle between them.

This enabled them to observe low-energy collisions between NH3-NH3 molecules and demonstrate that the energy-dependent changes in cross-section are different when substituting hydrogen atoms with deuterium (NH3-ND3 or ND3-ND3 collisions). Van de Meerakker notes that the decrease in cross sections at low energies might complicate experiments that require scattering to occur in crossed or merged beams.

However, it could benefit experiments where dipolar molecules are stored in traps, as lower inelastic cross-sections imply fewer molecules are lost from the trap. Moving forward, the team plans to introduce a controlled electric field into the beam overlap region. This mechanism is anticipated to respond sensitively to this field, allowing for significant changes in cross sections with small electric fields and potentially providing a "knob" to control collision outcomes, as envisioned by van de Meerakker.