On the atomic scale, even small differences such as a change in nuclear mass between isotopes can produce considerable quantum effects. Understanding the adsorption of small molecules (such as H₂) onto solid surfaces plays an important role in several areas, including hydrogen storage, heterogeneous catalysis, embrittlement, and spin isomer conversion. However, atomic-scale characterization of weakly physisorbed hydrogen remains challenging owing to low adsorption energies (typically 4–7 kJ mol⁻¹) in porous materials and the need for cryogenic conditions.

Writing in Physical Review Letters, Akitoshi Shiotari and colleagues from research institutes in Germany and Japan present direct spectroscopic evidence of isotope-dependent nuclear quantum behaviour in simple diatomic molecules using tip-enhanced Raman spectroscopy (TERS). The researchers investigated individual H₂ and D₂ molecules adsorbed on a silver surface at cryogenic temperatures of ~10 K, revealing how the differences in mass affect their vibrational properties when confined inside a nanometre-scale junction between the scanning tip and sample surface.

TERS merges the chemical sensitivity of Raman spectroscopy with the spatial resolution of scanning tunnelling microscopy (STM). In the work of Shiotari and colleagues, an STM tip is used both to manipulate individual molecules and to enhance the local electromagnetic field. This setup allows the researchers to detect rotational and vibrational modes of single H₂ and D₂ molecules. A confined electromagnetic field within the sub-nanometre plasmonic gap — called a picocavity — enhances Raman scattering and enables the direct detection of transitions typically inaccessible in weakly physisorbed systems.

Well-defined vibrational and rotational transitions were observed at 4 121 and 351 cm⁻¹ for H₂, and 2 967 and 169 cm⁻¹ for D₂, respectively. These values were measured at the STM set point (sample bias of 10 mV and tunnelling current of 1.0 nA). As the tip–surface distance was reduced by 200 pm from the relative reference position (defined as 30 pm above the STM set point for H₂ and 70 pm for D₂), the H₂ vibrational mode red-shifted by ≈33 cm⁻¹. Meanwhile, the vibrational mode of D₂ only shifted ≈2 cm⁻¹, which is within the experimental uncertainty (<10 cm⁻¹). This large isotopic contrast goes beyond standard mass-dependent models. The substantial redshift in H₂ is attributed to enhanced nuclear quantum delocalization, where the lighter hydrogen nucleus has a more spread-out wavefunction, which couples more strongly to the anharmonic component of the potential in the junction where the molecules are confined. In contrast, the vibrational wavefunction of heavier deuterium remains more localized and is less influenced by the surrounding field.

To interpret the experimental data, the researchers combined density functional theory (DFT), path-integral molecular dynamics, and a quantum anharmonic vibrational model. DFT confirmed the vertical molecular orientation of the molecules on the surface and supported the observed vibrational behaviour. At low temperatures, D₂ forms a denser physisorbed layer owing to its lower zero-point amplitude, while H₂ occupies a slightly higher position within the picocavity — closer to the field-enhanced region near the tip. A two-dimensional potential energy surface was used to model how the intramolecular stretching and vertical displacement interact. These calculations showed that field confinement selectively softens the H–H vibrational mode, while D₂ — which is less affected by the field — remains more rigid, pointing towards a density-driven, geometry-mediated mechanism that may help explain the isotope effects observed in low-temperature Raman spectra.

Overall, this study demonstrates how TERS can distinguish isotope-specific nuclear dynamics with high precision, providing valuable insight for surface spectroscopy, quantum sensing, isotope-selective separation, and the design of hydrogen storage materials. This approach could also be used to probe chemical reactivity at catalytically active sites, as well as to study molecular-scale phenomena relevant for surface chemistry and the development of future quantum devices.