Single-molecule infrared spectroscopy with scanning tunneling microscopy

Probing vibrations at the single-molecule level is essential for achieving
bond-specific chemical control. Here, we introduce a new measurement
scheme that integrates frequency-tunable infrared excitation with scanning
tunneling microscopy to characterize vibration-mediated nuclear motions of
single molecules. We first validated the technique by monitoring the
infrared-induced rotation of the ethynyl radical, and then applied it to
pyrrolidine, mapping its conformational dynamics. The resulting broadband
spectra captured fundamental vibrational modes together with rich overtone
and combination bands inaccessible by conventional methods, confirmed with
isotopic substitutions. Density functional theory calculations showed that
delocalized modes coupled with pyrrolidine ring puckering drive the
structural transition, revealing altered selection rules compared to
traditional infrared spectroscopy. This new experimental platform enables
probing of molecular vibrations and transformations with atomic precision.