Electric field gradient quadrupole Raman modes observed in plasmon-driven catalytic reactions revealed by HV-TERS

Electric field gradient quadrupole Raman modes are observed in plasmon-driven chemical reactions investigated with high vacuum tip-enhanced Raman spectroscopy (HV-TERS). TER spectra reveal that 4-nitrobenzenethiol (4NBT) catalytically dimerizes to dimercaptoazobenzene (DMAB) under an HV-TERS setup. More importantly, we find that the electric field gradient leads to strong enhancement of the infrared (IR)-active modes of DMAB. The observation of both the Raman-active and IR-active modes of DMAB provides spectral evidence for ultrasensitive chemical analysis.