  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.
