Unlocking Unified Framework to Measure Atomic-Scale Surface Motion

Researchers at the University of Cambridge and the ISIS Facility have announced a breakthrough in materials science by establishing a unified framework to determine how atoms and molecules move across surfaces. The study, published in Physical Review B, provides a long-awaited “common currency” for comparing surface diffusion across different materials and measurement techniques.

Bridging the Gap in Nanoscale Dynamics

Surface diffusion is a fundamental process that underpins everything from heterogeneous catalysis and material growth to the formation of ice in space. While Helium-3 spin-echo (HeSE) spectroscopy has long been the premier tool for observing these movements at picosecond timescales, its data was historically difficult to convert into standard tracer diffusion coefficients.

“The information provided by HeSE is inherently richer than a single coefficient, but this mismatch of observables hindered its integration into the broader scientific literature,” the researchers noted.

A Unified Workflow for Three Regimes

The new framework systematically distinguishes between three distinct types of atomic motion:

• Brownian Motion: Continuous movement occurring on flat energy landscapes.
• Hopping Motion: Discrete jumps between specific adsorption sites on a lattice.
• Correlated Motion: Complex behavior where atoms influence their neighbors, often seen in alkali metals like Sodium and Potassium on Copper surfaces.

By combining analytical models with Langevin molecular dynamics simulations, the team established a workflow that converts complex scattering data into diffusion constants​. This allows HeSE results to be directly compared with other techniques like scanning tunneling microscopy (STM) or neutron scattering.

Key Findings and Benchmark Library

The study also introduces the first initial benchmark library of HeSE-derived diffusion coefficients. Notable results include:

• Hydrogen on Platinum: Confirmed as a single-jump dominant system with a diffusion coefficient of 2.77×10^−9 m^2s^−1 at 220 K.
• Collective Interaction: Unlike other methods, this framework explicitly captures how adsorbates interact at finite coverages, providing a more realistic view of surface transport.
• Counter-intuitive Mobility: The team found that Potassium atoms diffuse faster than Sodium on Copper surfaces despite their larger mass, due to a lower energy barrier.

Future Impact

This work standardizes the analysis of surface dynamics, making the high-speed precision of helium scattering accessible to the wider academic and engineering communities. It paves the way for better modeling of chemical reactions and the fabrication of next-generation electronic devices.