Single-molecule infrared spectroscopy with scanning tunneling microscopy.

Probing vibrations at the single-molecule level is essential for achieving bond-specific chemical control in realistic heterogeneous environments. 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 mapping pyrrolidine's conformational dynamics. The resulting broadband spectra captured fundamental vibrational modes together with rich overtone and combination bands inaccessible by conventional methods, which we 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 with traditional infrared spectroscopy. This new experimental platform enables molecular vibrations and transformations to be probed with atomic precision. Editor's summary: Molecular vibrations play a crucial role in chemistry, and achieving vibrational spectroscopy at the single-molecule level has been a long-standing goal. This is especially true in the field of nanoscience, where the ability to manipulate chemical transformations with bond-level specificity is becoming increasingly important. By integrating frequency-tunable infrared (IR) excitation with scanning tunneling microscopy (STM), Liang et al. developed a broadband IR-STM platform able to resolve a series of fundamental vibrational modes in a single pyrrolidine molecule on the Cu(100) surface. They observed higher-order overtone and combination modes, as confirmed through isotopic substitution. These results demonstrate that IR-STM can provide a more comprehensive molecular vibrational fingerprint compared with previously proposed techniques for vibrational characterization at the single-molecule level. —Yury Suleymanov [ABSTRACT FROM AUTHOR]