Activated Vibrational Modes and Fermi Resonance in Tip-Enhanced Raman Spectroscopy

TL;DR

This study demonstrates how tip-enhanced Raman spectroscopy (TERS) can activate vibrational modes and reveal Fermi resonance effects in molecules like PATP on gold, providing detailed spectral insights and evidence of chemical transformations.

Contribution

It shows the first detailed assignment of IR active modes in TERS and the observation of Fermi resonance effects, expanding TERS capabilities for molecular spectroscopy.

Findings

IR active modes are activated by electromagnetic field gradients.

Fermi resonance causes splitting of vibrational modes in TERS.

Spectroscopic evidence of PATP to DMAB conversion.

Abstract

Using p-aminothiophenol (PATP) molecules on a gold substrate as prototypical examples and high vacuum tip-enhanced Raman spectroscopy (HV-TERS), we show that the vibrational spectra of those molecules are distinctly different from those in typical surface-enhanced Raman spectroscopy. Detailed first-principles calculations help to assign the Raman peaks in the TERS measurements as Raman active and infrared (IR) active vibrational modes of dimercaptoazobenzene (DMAB), thus providing strong spectroscopic evidence for the conversion of PATP dimerization to DMAB. The activation of the IR active modes is due to enhanced electromagnetic field gradient effects within the gap region of the highly asymmetric tip-surface geometry. Our TERS measurements also realize splitting of certain vibrational modes due to Fermi resonance between a fundamental mode and the overtone of a different mode or a combinational mode. These findings help to broaden the versatility of TERS as a promising technique for ultrasensitive molecular spectroscopy.