Tip‐Enhanced Resonance Couplings Revealed by High Vacuum Tip‐Enhanced Raman Spectroscopy

Tip‐enhanced resonance couplings (TERCs) are experimentally revealed by high vacuum tip‐enhanced Raman spectroscopy (HV‐TERS) and theoretically interpreted. The Fermi resonance and Darling–Dennison resonance are successfully observed in HV‐TERS, which are the first overtone (or combinational mode)‐fundamental interaction (2:1 resonance coupling) and the first overtone‐first overtone resonant interaction (1:1 resonance coupling), respectively. The electric field gradient plays an important role on TERCs in HV‐TERS at the level of the second‐order perturbation theory. The molecular tautomeric effects are also observed from thiourea adsorbed on Ag film in HV‐TERS. The reported HERCs can provide a deeper understanding of the importance of molecular anharmonicity in high‐order perturbation for TERS. HV‐TER spectra provide ‘additional’ nonlinear Raman peaks, compared with the harmonic zero‐order perturbation in HV‐TERS, and thus enable ultrasensitive chemical analysis at the nanoscale with more vibrational information.