Submolecular‐Resolution Probing of Vibrational Anharmonicity Using Tip‐Enhanced Raman Spectroscopy

Abstract

Abstract Vibrational spectroscopy can reach atomic spatial resolution via highly confined optical probes, offering local chemical insight. Yet, overtones and combination bands have not fully benefited due to weak transition moments. Here, we show submolecular‐resolution detection of intense overtones and combination bands using tip‐enhanced Raman spectroscopy (TERS) for an asymmetric perylene derivative on silicon. The Raman peaks, barely visible in tunneling regime, are ∼10 times enhanced when the tip‐apex contacts the molecule. This point‐contact‐mode TERS enables analysis of two anharmonicities with submolecular contrast: one arising from deviations of potential energy surfaces from harmonic shape (mechanical anharmonicity), the other from high‐order polarizability derivatives (electrical anharmonicity). Spatial variation of mechanical anharmonicity reveals a vibrational energy exchange channel at the submolecular scale, highlighting potential to map energy transfer in real space.