Molecular hydrogen is a key renewable energy material, and its adsorption on solid surfaces is important for its storage, transport, and catalytic reactions. Characterizing the structure and dynamics of these small and fast-diffusing molecules weakly adsorbed (physisorbed) on metallic surfaces has long been a major challenge. We investigate hydrogen molecules on a silver surface and demonstrate that low-temperature tip-enhanced Raman spectroscopy (LT-TERS) reveals the vibrational and rotational properties of the physisorbed molecule at the local tip–molecule–surface junction. For LT-TERS a silver probe tip is illuminated by a visible laser, causing plasmonic resonance to create a ultraconfined electric field on the (1 nm)³ scale in a plasmonic picocavity which significantly enhances the intrinsically small Raman scattering efficiency. We observe an enormously large isotope effect in the vibrational frequencies between hydrogen (H₂) and deuterium (D₂) in the junction. Theoretical calculations show that the nuclear quantum effects on the molecular density on the surface cause the distinctive isotope effect. The state-of-the-art LT-TERS technique will contribute to a better microscopic understanding of physical and chemical properties of hydrogen and other physisorption systems.