Picocavity-Enhanced Raman Spectroscopy of Physisorbed H$_2$ and D$_2$ Molecules

TL;DR

This study demonstrates picocavity-enhanced Raman spectroscopy of H2 and D2 molecules at cryogenic temperatures, revealing isotope-dependent vibrational shifts influenced by molecular density within a plasmonic nanogap.

Contribution

It introduces a novel application of tip-enhanced Raman spectroscopy to physisorbed hydrogen molecules, highlighting an unexpected isotope effect explained by molecular density differences.

Findings

Redshift of H-H stretch frequency with decreasing gap

D-D stretch frequency remains unaffected by gap changes

Density functional theory explains isotope effect via molecular density differences

Abstract

We report on tip-enhanced Raman spectroscopy of H2 and D2 molecules physisorbed within a plasmonic picocavity at 10 K. The intense Raman peaks resulting from the rotational and vibrational transitions are observed at subnanometer gap distances of the junction formed by an Ag tip and an Ag(111) surface, where a picocavity-enhanced field plays a crucial role. A significant redshift of the H-H stretch frequency is observed as the gap distance decreases, while the D-D stretch frequency is unaffected. Density functional theory, path-integral molecular dynamics, and quantum anharmonic vibrational energy calculations suggest that this unexpected isotope effect is explained by a different molecular density between H2 and D2 on the surface.