Picometric Spectroscopy of Hydrogen Molecule in Atomic-Scale Cavity
An international research team, led by Akitoshi Shiotari of the Fritz Haber Institute of the Max Planck Society (FHI, Germany), Mariana Rossi of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD, Germany) and Takashi Kumagai of the Institute for Molecular Science/SOKENDAI (IMS, Japan) has successfully achieved the single-molecule spectroscopic observation of hydrogen (H2) and deuterium (D2) confined within a picocavity. The picocavity was formed between a silver nanotip and a silver single-crystal substrate under cryogenic and ultrahigh vacuum conditions, using tip-enhanced Raman spectroscopy (TERS).
In recent years, light–matter interactions within atomic-scale volumes, known as picocavities, have attracted growing attention in nanoscience and nanotechnology. The extremely confined plasmonic field due to the 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 in a picocavity and investigated by high-resolution TERS. This approach enabled picometric molecular spectroscopy to observe its vibrational and rotational modes with unprecedented detail, successfully revealing how the structure and vibrational properties of a single molecule are affected by the extreme spatial confinement of the picocavity. 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, showed a significant change, demonstrating a large isotope-dependent effect — that could not be captured by ensemble-averaged Raman or any other conventional vibrational spectroscopies.
To elucidate the origin of this nontrivial isotope effect, the team conducted advanced theoretical simulations combining density-functional theory (DFT), path-integral molecular dynamics (PIMD) and model Hamiltonians. These calculations revealed that this spectroscopy is exquisitely sensitive to the local interaction potential felt 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 different positions for H2 and D2 in the picocavity, which result in an enormous 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 says, “Looking ahead, the methods and insights developed here are expected to contribute to the advanced analysis of hydrogen storage materials and catalytic reactions and to the development of quantum control technologies for individual molecules, thus supporting next-generation nanoscale sensing and quantum photonic technologies.”
