Experimental evidence is given of the breakdown of Raman selection rules in high vacuum tip-enhanced Raman spectroscopy (HV-TERS), where molecular Raman-active, IR-active, and overtone modes are simultaneously observed in situ, due to the strong tip-enhanced near-field gradient effects. Theoretical calculations firmly support our experimental multipole vibrational observation, and the ratio of the near-field gradient term over electric field term is dependent on both the ratio of and the ratio of , which are dependent on the physical structure of TERS and the molecular structure, respectively, in HV-TERS. When the molecule is symmetric along the tip axis, the strongest near-field gradient effect in HV-TERS can be obtained. These experimental findings can promote an understanding of the unexpected “additional Raman peaks” in HV-TERS and provide a promising technique for ultrasensitive molecular spectroscopy.

We successfully realized in-situ monitoring plasmon-driven selective reduction of 2,4-dinitrobenzenethiol to 2,2′-diamino-dimercaptoazobenzene, revealed by high vacuum tip-enhanced Raman spectroscopy (HV-TERS). The HV-TER spectra revealed that the 2-nitro and the 4-nitro of 2,4-DNBT were selectively reduced to the 2-amine and the –N = N– bond of 2,2′-diamino-dimercaptoazobenzene (2,2′DA-DMAB). Raman-active and IR-active modes as well as Fermi resonance were simultaneously observed in HV-TERS, which demonstrated the advantages of HV-TERS over SERS, since only Raman-active modes were observed in SERS. The intensities of molecular IR-active modes can be manipulated by the distance between tip and substrate in the near field, due to different dependences of the plasmon gradient and plasmon intensity over the distance of nano gap. Our results in HV-TERS are in-situ “complete-vibration modes” spectral analysis, which significantly extend the application of HV-TERS in the field of ultrasensitive spectral analysis on the nano scale.

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.

The one-dimensional (1D) transition-metal oxide MoO₃ belt is synthesized and characterized with X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. Charge-transfer-(CT) enhanced Raman scattering of 4-mercaptobenzoic acid (4-MBA) on a 1D MoO₃ belt was investigated experimentally and theoretically. The chemical enhancement of surface-enhanced Raman scattering (SERS) of 4-MBA on the MoO₃ belt by CT is in the order of 10³. The SERS of 4-MBA was investigated theoretically by using a quantum chemical method. The remote SERS of 4-MBA along the 1D MoO₃ belt (the light excitation to one side of the MoO₃ belt, and the SERS spectrum is collected on the other side of the MoO₃ belt) is also shown experimentally, which provides potential applications of SERS. The incident polarization dependence of remote SERS spectra has also been investigated experimentally.