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Plasmonic Gradient Effects on High Vacuum Tip-Enhanced Raman Spectroscopy



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 math formula and the ratio of math formula, 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.