Picometric spectroscopy of hydrogen molecules in atomic-scale cavities

In recent years, light-matter interactions within atomic-scale volumes, known as picocavities, have attracted growing attention in nanoscience and nanotechnology. The extremely confined electromagnetic field generated by plasmon resonance is now regarded as a promising platform for atomic-scale measurements and quantum photonic technologies.

In this study, the smallest molecule -- hydrogen -- was confined within a picocavity and investigated using high-resolution TERS. This enabled picometric molecular spectroscopy to resolve its vibrational and rotational modes with unprecedented detail, revealing how the structure and vibrational properties of a single molecule are affected by the extreme spatial confinement of the picocavity. Furthermore, by precisely adjusting the gap distance between the silver tip and the silver substrate, the subtle interaction with the molecule is modified. As a result, it was discovered that only the vibrational mode of H₂, and not D₂, exhibited a significant change, demonstrating a pronounced isotope-dependent effect -- that could not be captured by ensemble-averaged Raman or other conventional vibrational spectroscopies.

To elucidate the origin of this nontrivial isotope effect, the team conducted theoretical simulations using density functional theory (DFT), path-integral molecular dynamics (PIMD), and model Hamiltonians. These calculations revealed that the spectroscopy is exquisitely sensitive to the local interaction potential experienced by the molecules, dominated by van der Waals interactions. Quantum delocalization of the nuclei -- a quantum swelling effect at low temperatures -- plays a decisive role in the observed differences, favoring distinct equilibrium positions for H₂ and D₂ in the picocavity, which lead to a substantial difference in their vibrational spectra. Dr. Rossi says, "We were surprised at how vibrational coupling and nuclear quantum effects work hand-in-hand to cause such a large isotope effect."

Dr. Shiotari says, "This work deepens our understanding of light-molecule interactions and the quantum dynamics of adsorbed molecules in extremely confined spaces, representing a significant step forward in precision molecular spectroscopy." Prof. Kumagai adds, "Looking ahead, the methods and insights developed here are expected to contribute to the advanced analysis of hydrogen storage materials and catalytic reactions, as well as to the development of quantum control technologies for individual molecules -- thereby supporting next-generation nanoscale sensing and quantum photonic technologies."