Molecules inside superfluid helium nanodroplets have been driven beyond the point where the surrounding fluid was expected to stop their rotation, according to a new laboratory study. CS2 followed the rotating drive up to about 80 gigahertz, far beyond its approximately 22 GHz centrifugal wall. OCS also passed its 36 GHz wall, although its alignment signal weakened sooner and more quickly.
The work suggests that the loss of molecular alignment can help compare how strongly different molecules couple to the helium environment. The authors’ calculations gave CS2 an effective thermalization time of 10 picoseconds and OCS a time of 100 picoseconds, a tenfold difference.
A rotating drive, read out from fragments
The experiment used an ultraslow optical centrifuge to drive CS2 and OCS in helium nanodroplets. Molecular rotation was detected through Coulomb explosion and Velocity Map Imaging, which measures the distribution of ion fragments after the molecule is broken apart.
The droplets were produced at a pressure of 30 bar and at 16 K or 18 K, depending on the target molecule. They were expected to contain roughly 3,000 to 5,000 helium atoms and were doped so that each droplet held at most one CS2 or OCS molecule.
For each ion hit, the researchers evaluated the fragment angle in the detector plane and averaged the resulting alignment measure over several thousand ions. They repeated the measurement while changing the probe’s arrival time relative to the centrifuge, building a time-dependent picture of how the molecules stayed oriented.
The alignment signal changed with the molecular rotor
For CS2, the frequency pattern in the measured signal tracked the driven rotation to about 80 GHz, despite the centrifugal wall near 22 GHz. To follow that changing frequency, the analysis resampled the alignment trace onto a uniform grid and applied a short-time Fourier transform using a 180 ps Blackman window.
OCS showed the same broad ability to rotate beyond its approximately 36 GHz wall. But the oscillations associated with angular localization began to lose amplitude earlier than they did for CS2, and the decline was faster. When the centrifuge was decelerated, the pattern reversed: level splittings became smaller, angular localization returned, and the oscillation amplitude grew.
After the pulse, the alignment observable for both molecules returned shortly afterward to 0.5, the isotropic baseline reported in the study.
What the fitted times mean
The researchers compared the measurements with numerical models of a rotor interacting with its environment. The CS2 signal was best reproduced with an effective thermalization time of 10 ps at high rotational frequency. For OCS, a calculation using 100 ps reproduced the faster fall in alignment amplitude.
The authors interpret this pattern as rapid thermalization by the helium bath helping sustain rotation, while increasing level splittings at higher rotation frequencies dephase the aligned state restored by the bath. On that reading, the frequency at which alignment is lost becomes a probe of molecule-specific bath coupling, and the fitted times differ by an order of magnitude between the two molecules.
The main comparison used an instantaneous steady-state calculation. The rotating-frame Hamiltonian was assembled and fully diagonalized at 500 uniformly spaced time steps. The study also used time-dependent Lindblad propagation to retain transient dynamics omitted by that steady-state construction.
A promising probe with important limits
The thermalization times should be read as effective parameters within the study’s modeling framework. The authors describe the assumption of one thermalization time for every rotational state as crude and call for a state-resolved treatment of molecule-bath coupling. The reported 10 ps and 100 ps values therefore summarize the model’s account of the observed alignment dynamics.
The study’s status
The supplied document is an arXiv version 1 preprint dated 25 August 2026. The research acknowledgments list support from CFI, BCKDF, NSERC, Villum Fonden, the European Union Regional Development Fund through the stated project, the NOMIS Foundation, and the Austrian Academy of Science ÖAW.
Paper data and sources
Original title: Optical centrifuge as a probe of strong dissipative coupling between a molecular rotor and superfluid helium
Authors: Ian MacPhail-Bartley, Sören E. Mahr, Cameron E. Peters et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-25
DOI: Not available
Original paper · Full text