Seeing a Molecule’s Quantum Shadow
A century ago, physicists showed that particles could act like waves, described mathematically by a wave function. The wave function’s spatial extent depends on the particle’s mass and environment. Now Wenbin Zhang at East China Normal University and his colleagues have developed an optical method that sheds light on this dependence for a single molecule trapped in a superfluid helium nanodroplet [1]. The approach could help scientists interpret experiments on confined molecules and design nanoscale devices that harness such particles.
Zhang and his colleagues used individual molecules of either hydrogen or its heavier cousin deuterium. They trapped each molecule in a separate 2-nm-sized superfluid helium nanodroplet. The temperature of these droplets meant that their spatial extent would be similar to that of the molecules’ wave functions. The researchers then used a laser to knock out one electron from each molecule. The ejected electrons traveled through the droplets and hit a detector, which recorded each electron’s momentum. Lastly, the team analyzed patterns formed in the distribution of these momenta for each molecule type.
For the deuterium molecules, these patterns were substantially blurred, suggesting that the electrons frequently scattered from the droplets’ atoms on their journey to the detector. This finding showed that the wave functions of these heavier molecules were smaller than the droplet size of 2 nm. For the hydrogen molecules, the patterns were clear and sharp, implying that the electrons escaped the droplets with minimal scattering. This result would only be possible if the wave functions of these lighter molecules were about 2 nm.
In a related work, Zhang and his colleagues have also explored how such a droplet affects a fundamental reaction of an enclosed hydrogen molecule [2].
–Ryan Wilkinson
Ryan Wilkinson is a Corresponding Editor for Physics Magazine based in Durham, UK.
References
- Z. Ye et al., “Localization and delocalization of a single molecule in a helium nanodroplet,” Phys. Rev. Lett. 135, 213202 (2025).
- Z. Ye et al., “Laser-induced Coulomb explosion dynamics of H2 molecules in helium nanodroplets,” Phys. Rev. A 112, 063118 (2025).