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May 21, 2025

Single-molecule spectroscopy reveals isotope effect in hydrogen confined to picocavities

Raman scattering of a hydrogen molecule in a plasmonic picocavity. Credit: Takashi Kumagai
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Raman scattering of a hydrogen molecule in a plasmonic picocavity. Credit: Takashi Kumagai

An international research team has successfully achieved 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).

The research is in the journal Âé¶¹ÒùÔºical Review Letters. The team was led by Akitoshi Shiotari of the Fritz Haber Institute of the Max Planck Society (Germany), Mariana Rossi of the Max Planck Institute for the Structure and Dynamics of Matter (Germany), and Takashi Kumagai of the Institute for Molecular Science/SOKENDAI (Japan).

In recent years, within atomic-scale volumes, known as picocavities, have attracted growing attention in nanoscience and nanotechnology. The extremely confined electromagnetic field generated by 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.

Credit: Âé¶¹ÒùÔºical Review Letters (2025). DOI: 10.1103/Âé¶¹ÒùÔºRevLett.134.206901
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Credit: Âé¶¹ÒùÔºical Review Letters (2025). DOI: 10.1103/Âé¶¹ÒùÔºRevLett.134.206901

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.

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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."

More information: Akitoshi Shiotari et al, Picocavity-Enhanced Raman Spectroscopy of Âé¶¹ÒùÔºisorbed H2 and D2 Molecules, Âé¶¹ÒùÔºical Review Letters (2025).

Journal information: Âé¶¹ÒùÔºical Review Letters

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Single-molecule tip-enhanced Raman spectroscopy of hydrogen and deuterium confined in a picocavity reveals a pronounced isotope-dependent vibrational response, with only H2 showing significant spectral changes under spatial confinement. This effect arises from quantum nuclear delocalization and van der Waals interactions, highlighting the sensitivity of molecular properties to extreme nanoscale environments.

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